Below is a collection of Ahadith in which the Imams (as) have advised/warned against rejection of Ahadith attributed to them.

Hadith 1:

In a lengthy hadith, Imam Moosa Kazim (as) says the following in one of his letters,

ووال آل محمد ولا تقل لما بلغك عنا ونسب إلينا هذا باطل وإن كنت تعرف منا خلافه فإنك لا تدري لما قلناه وعلى أي وجه وصفناه 

"Accept wilayah of Ale (family of) Mohammed and do not say about what has reached you from us or is ascribed to us as false even though you may know something from Us which is against it, because you do not know why we said it and how we explained it."

[Source: Al-Kafi. Vol. 8, Pg. 124, H. 95]

Hadith 2:

أَبِي (رحمه الله) قَالَ حَدَّثَنَا سَعْدُ بْنُ عَبْدِ اللَّهِ عَنْ أَحْمَدَ بْنِ أَبِي عَبْدِ اللَّهِ عَنْ مُحَمَّدِ بْنِ إِسْمَاعِيلَ بْنِ بَزِيعٍ عَنْ جَعْفَرِ بْنِ بَشِيرٍ عَنْ أَبِي حُصَيْنٍ عَنْ أَبِي بَصِيرٍ عَنْ أَحَدِهِمَا (ع) قَالُوا لَا تُكَذِّبُوا بِحَدِيثٍ أَتَاكُمْ بِهِ مُرْجِئِيٌّ وَ لَا قَدَرِيٌّ وَ لَا خَارِجِيٌّ نَسَبَهُ إِلَيْنَا فَإِنَّكُمْ لَا تَدْرُونَ لَعَلَّهُ شَيْءٌ مِنَ الْحَقِّ فَتُكَذِّبُوا اللَّهَ عَزَّ وَ جَلَّ فَوْقَ عَرْشِهِ

From Abu Basir, from one of the two Imams (as), who said,

"Do not deny a hadith that reaches to you through a Murji, a Qadri or a khariji that he attributes to Us, since you would not know that it could be from the truth and hence you would result in having denied Allah above His throne. "

[Source: Ilal al-Shara'i, Shaikh Sadooq, Vol. 2, Pg. 395]

Hadith 3:

حَدَّثَنَا أَحْمَدُ بْنُ مُحَمَّدِ بْنِ عِيسَى، عَنِ الْحُسَيْنِ بْنِ سَعِيدٍ وَ مُحَمَّدِ بْنِ خَالِدٍ الْبَرْقِيِّ، عَنْ عَبْدِ اللَّهِ بْنِ جُنْدَبٍ، عَنْ سُفْيَانَ بْنِ السِّمْطِ، قَالَ: قُلْتُ لِأَبِي عَبْدِ اللَّهِ ع: جُعِلْتُ فِدَاكَ يَأْتِينَا الرَّجُلُ مِنْ قِبَلِكُمْ يُعْرَفُ بِالْكَذِبِ فَيُحَدِّثُ بِالْحَدِيثِ فَنَسْتَبْشِعُهُ.

فَقَالَ أَبُو عَبْدِ اللَّهِ ع: «يَقُولُ لَكَ: إِنِّي قُلْتُ الليل [لِلَّيْلِ‌] إِنَّهُ نَهَارٌ، وَ النهار [لِلنَّهَارِ] إِنَّهُ لَيْلٌ؟» قُلْتُ: لَا، قَالَ: «فَإِنْ قَالَ لَكَ هَذَا إِنِّي قُلْتُهُ فَلَا تُكَذِّبْ بِهِ، فَإِنَّكَ إِنَّمَا تُكَذِّبُنِي

From Sufyan bin Samt, who said: I said to Imam al-Sadiq (as), "May I be sacrificed for you. Sometimes someone known for lying comes to us and narrates a Hadith, after which we are disgusted." 

Imam al-Sadiq (as) said: "Did he say to you that I said that the night is the day and that the day is night?" 

I said: "No."

Imam (as) said, "If he says to that I have said this, then don't deny it , or you will as a result deny me."

[Source: Mukhtasar Basair al-Darajaat, Pg. 233]

Hadith 4:

عَنْهُ عَنْ أَحْمَدَ بْنِ مُحَمَّدٍ عَنِ ابْنِ مَحْبُوبٍ عَنْ جَمِيلِ بْنِ صَالِحٍ عَنْ أَبِي عُبَيْدَةَ الْحَذَّاءِ قَالَ سَمِعْتُ أَبَا جَعْفَرٍ (عَلَيهِ السَّلام) يَقُولُ وَالله إِنَّ أَحَبَّ أَصْحَابِي إِلَيَّ أَوْرَعُهُمْ وَأَفْقَهُهُمْ وَأَكْتَمُهُمْ لِحَدِيثِنَا وَإِنَّ أَسْوَأَهُمْ عِنْدِي حَالاً وَأَمْقَتَهُمْ لَلَّذِي إِذَا سَمِعَ الْحَدِيثَ يُنْسَبُ إِلَيْنَا وَيُرْوَى عَنَّا فَلَمْ يَقْبَلْهُ اشْمَأَزَّ مِنْهُ وَجَحَدَهُ وَكَفَّرَ مَنْ دَانَ بِهِ وَهُوَ لا يَدْرِي لَعَلَّ الْحَدِيثَ مِنْ عِنْدِنَا خَرَجَ وَإِلَيْنَا أُسْنِدَ فَيَكُونَ بِذَلِكَ خَارِجاً عَنْ وَلايَتِنَا

Narrated that Abu Jafar Imam Baqir (ع) said,

"By Allah, the most beloved to Me of My companions are most restraining from the worldly attractions, most knowledgeable in Fiqh (laws of Sharia) and most secretive in our Hadith. Of the worst conditions among them and most disliked is the one who upon hearing the Hadith that is ascribed it to Us and narrated it from Us, does not accept it, detesting it and rejecting it and considering as disbelievers those to accept the Hadith, while he does not know that perhaps the Hadith has come out from Us and is traced back to Us and as a result of this he will be out of our Walayah (guardianship)."

[Source: al-Kafi, Vol. 2, Pg. 223]

Hadith 5:

In this hadith Imam Sadiq (as) is seen praising a person who submits to everything that comes from the Imams (as).

مُحَمَّدُ بْنُ يَحْيَى عَنْ أَحْمَدَ بْنِ مُحَمَّدٍ عَنِ الْحُسَيْنِ بْنِ سَعِيدٍ عَنْ حَمَّادِ بْنِ عِيسَى عَنِ الْحُسَيْنِ بْنِ الْمُخْتَارِ عَنْ زَيْدٍ الشَّحَّامِ عَنْ أَبِي عَبْدِ الله (عَلَيْهِ السَّلام) قَالَ قُلْتُ لَهُ إِنَّ عِنْدَنَا رَجُلاً يُقَالُ لَهُ كُلَيْبٌ فَلا يَجِي‏ءُ عَنْكُمْ شَيْ‏ءٌ إِلا قَالَ أَنَا أُسَلِّمُ فَسَمَّيْنَاهُ كُلَيْبَ تَسْلِيمٍ قَالَ فَتَرَحَّمَ عَلَيْهِ ثُمَّ قَالَ أَ تَدْرُونَ مَا التَّسْلِيمُ فَسَكَتْنَا فَقَالَ هُوَ وَالله الاخْبَاتُ قَوْلُ الله عَزَّ وَجَلَّ الَّذِينَ آمَنُوا وَعَمِلُوا الصَّالِحاتِ وَأَخْبَتُوا إِلى‏ رَبِّهِمْ

Narrated from Zayd al-Shahham who said, 

“I said to Abu Abdallah Imam Sadiq (as), ‘Among us there is a man called Kulayb. Nothing comes from you but that he says, “I submit to it.” Thus, we call him, “Kulayb, the submissive.” 

The Imam (as) prayed for Mercy on him (Tarahham) and then the Imam (as) said, “Do you know what submission is?”

We remained silent. The Imam (as) said, “That, by Allah, is al-Ikhbat (humbleness) in the words of Allah (عز و جل), "Those who believe and do righteous deeds and are humble before their Lord." (11:23)

[Source: al-Kafi, Vol.1, Pg. 390-391, Hadith. 3]

Hadith 6:

مُحَمَّدُ بْنُ يَحْيَى عَنْ أَحْمَدَ بْنِ مُحَمَّدِ بْنِ عِيسَى عَنْ مُحَمَّدِ بْنِ سِنَانٍ عَنْ نَصْرٍ الْخَثْعَمِيِّ قَالَ سَمِعْتُ أَبَا عَبْدِ اللَّهِ ع يَقُولُ‌ مَنْ عَرَفَ أَنَّا لَا نَقُولُ إِلَّا حَقّاً فَلْيَكْتَفِ بِمَا يَعْلَمُ مِنَّا فَإِنْ سَمِعَ مِنَّا خِلَافَ مَا يَعْلَمُ فَلْيَعْلَمْ أَنَّ ذَلِكَ دِفَاعٌ مِنَّا عَنْهُ‌

Narrated that Abu Abd-Allah Imam Sadiq (عليه السلام) said,

"Whoever knows that We say nothing but the truth, then he should be content/satisfied with what he knows is from Us. And if he hears from Us something that contradicts what he knows, then he should know that it is Our way of protecting him."

[Source: al-Kafi, Vol. 1, Pg. 65-66]

Hadith 7:

جِبْرِيلُ بْنُ أَحْمَدَ الْفَارَيَابِيُّ، حَدَّثَنِي مُحَمَّدُ بْنُ عِيسَى الْعُبَيْدِيُّ، عَنْ عَلِيِّ بْنِ حَسَّانَ الْهَاشِمِيِّ، قَالَ حَدَّثَنِي عَبْدُ الرَّحْمَنِ بْنُ كَثِيرٍ، عَنْ جَابِرِ بْنِ يَزِيدَ قَالَ قَالَ أَبُو جَعْفَرٍ (ع) يَا جَابِرُ حَدِيثُنَا صَعْبٌ مُسْتَصْعَبٌ، أَمْرَدُ ذَكْوَانُ‌ وَعْرٌ أَجْرَدُ لَا يَحْتَمِلُهُ وَ اللَّهِ إِلَّا نَبِيٌّ مُرْسَلٌ أَوْ مَلَكٌ مُقَرَّبٌ أَوْ مُؤْمِنٌ مُمْتَحَنٌ، فَإِذَا وَرَدَ عَلَيْكَ يَا جَابِرُ شَيْ‌ءٌ مِنْ أَمْرِنَا فَلَانَ لَهُ قَلْبُكَ فَاحْمَدِ اللَّهَ، وَ إِنْ أَنْكَرْتَهُ فَرُدُّوهُ إِلَيْنَا أَهْلَ الْبَيْتِ، وَ لَا تَقُلْ كَيْفَ جَاءَ هَذَا! وَ كَيْفَ كَانَ وَ كَيْفَ هُوَ! فَإِنَّ هَذَا وَ اللَّهِ الشِّرْكُ بِاللَّهِ الْعَظِيمِ

Narrated from Jabir ibn Yazid, that Abu Jafar Imam Baqir (عليه السلام) said, "O Jabir, Our narration is difficult to bear and is found to be difficult. By Allah, only a Messenger Prophet, a close angel, or a tested believer can endure it. 

So when something from Our affair/command/hadith comes to you, O Jabir, and your heart agrees/yields (becomes soft/submissive) to it, then praise Allah. And if you (want to) reject it, then refer it back to Us - the AhlulBayt (عليهم السلام). And don't say, 'How did this come? How is it?' For, by Allah, this will be associating partners (shirk) with Allah (العظيم)."

[Source: Rijal al-Kashshi, Vol. 1, Pg. 193-194]

Hadith 8:

محمد بن يحيى، عن محمد بن الحسين، عن محمد بن سنان، عن عمار بن مروان عن جابر قال قال أبو جعفر عليه السلام: قال رسول الله صلى الله عليه وآله إن حديث آل محمد صعب مستصعب لا يؤمن به إلا ملك مقرب أو نبي مرسل أو عبد امتحن الله قلبه للايمان، فما ورد عليكم من حديث آل محمد صلى الله عليه وآله فلانت له قلوبكم وعرفتموه فاقبلوه، وما اشمأزت منه قلوبكم وأنكرتموه فردوه إلى الله وإلى الرسول وإلى العالم من آل محمد وإنما الهالك أن يحدث أحدكم بشئ منه لا يحتمله، فيقول: والله ما كان هذا والله ما كان هذا، والانكار هو الكفر

Narrated from Abu Jafar Imam Baqir (عليه السلام), that the Messenger of Allah (صلى الله عليه وآله) has said, 

"Surely the narration (hadith) of Aal-e-Muhammad (عليهم السلام) is difficult is and is found to be difficult. No one believes in it except the angels that are close to Allah or the Prophets who are Messengers also, or a servant whose heart Allah has tested for faith.

So whatever would come to you of the Hadith (narration) of Aal-e-Muhammad (عليهم السلام), and your heart agrees/yields (becomes soft/submissive) to it and you recognize it, then accept it. And any hadith which your hearts detest and you reject it, then return it to Allah, to the Messenger of Allah (ص) and the scholar from Aal-e-Muhammad (عليهم السلام). 

The one who will perish is the one to whom something from our Ahadith is narrated which he cannot bear, so he says - 'By Allah, this did not happen! By Allah, this did not happen!' And rejection (of hadith) is disbelief (kufr)."

[Source: al-Kafi, Vol. 1, Pg. 401]