حَدَّثَنا مُحَمَّدِ بْنِ إِبْراهيمِ بْنِ إِسْحاق الطَّالِقانِيُّ رَضِىَ اللهُ عَنْهُ قالَ: حَدَّثَنا أَحْمَدِ بْنِ مُحَمَّدِ بْنِ سَعِيدُ الكُوفِي قالَ: حَدَّثَنا عَلِىِّ بْنِ الحَسَنِ بْنِ عَلِىِّ بْنِ فضال، عَن أَبيهِ قالَ: سَأَلْتُ الرِّضَا عَلَيْهِ السَّلامُ فَقُلْتُ لَهُ لِمَ كُنِّيَ النَّبِيُ‏ صَلَّى اللهُ عَلَيْهِ وَآلِهِ بِأَبِي الْقَاسِمِ فَقَالَ لانَّهُ كَانَ لَهُ ابْنٌ يُقَالُ لَهُ قَاسِمٌ فَكُنِّيَ بِهِ قَالَ فَقُلْتُ يَا ابْنَ رَسُولِ اللَّهِ فَهَلْ تَرَانِي أَهْلاً لِلزِّيَادَةِ فَقَالَ نَعَمْ أَمَا عَلِمْتَ أَنَّ رَسُولَ اللَّهِ‏ صَلَّى اللهُ عَلَيْهِ وَآلِهِ قَالَ أَنَا وَعَلِيٌّ أَبَوَا هَذِهِ الأُمَّةِ قُلْتُ بَلَى قَالَ أَمَا عَلِمْتَ أَنَّ رَسُولَ اللَّهِ‏ صَلَّى اللهُ عَلَيْهِ وَآلِهِ أَبٌ لِجَمِيعِ أُمَّتِهِ وَعَلِيٌّ بِمَنْزِلَتِهِ فِيهِمْ قُلْتُ بَلَى قَالَ أَمَا عَلِمْتَ أَنَّ عَلِيّاً قَاسِمُ الْجَنَّةِ وَالنَّارِ قُلْتُ بَلَى قَالَ فَقِيلَ لَهُ أَبُو الْقَاسِمِ لانَّهُ أَبُو قَاسِمِ الْجَنَّةِ وَالنَّارِ فَقُلْتُ لَهُ وَمَا مَعْنَى ذَلِكَ فَقَالَ إِنَّ شَفَقَةَ الرَّسُولِ عَلَى أُمَّتِهِ شَفَقَةُ الآْبَاءِ عَلَى أَلأَوْلادِ وَأَفْضَلُ أُمَّتِهِ عَلِيٌ‏ عَلَيْهِ السَّلامُ وَمِنْ بَعْدِهِ شَفَقَةُ عَلِيٍ‏ عَلَيْهِ السَّلامُ عَلَيْهِمْ كَشَفَقَتِهِ لانَّهُ وَصِيُّهُ وَخَلِيفَتُهُ وَالإِمَامُ بَعْدَهُ فَلِذَلِكَ قَالَ‏ صَلَّى اللهُ عَلَيْهِ وَآلِهِ أَنَا وَعَلِيٌّ أَبَوَا هَذِهِ الأُمَّةِ وَصَعِدَ النَّبِيُ‏ صَلَّى اللهُ عَلَيْهِ وَآلِهِ الْمِنْبَرَ فَقَالَ مَنْ تَرَكَ دَيْناً أَوْ ضَيَاعاً فَعَلَيَّ وَإِلَيَّ وَمَنْ تَرَكَ مَالاً فَلِوَرَثَتِهِ فَصَارَ بِذَلِكَ أَوْلَى بِهِمْ مِنْ آبَائِهِمْ وَأُمَّهَاتِهِمْ وَصَارَ أَوْلَى بِهِمْ مِنْهُمْ بِأَنْفُسِهِمْ وَكَذَلِكَ أَمِيرُ الْمُؤْمِنِينَ‏ عَلَيْهِ السَّلامُ بَعْدَهُ جَرَى لَهُ مِثْلُ مَا جَرَى لِرَسُولِ اللَّهِ‏ صَلَّى اللهُ عَلَيْهِ وَآلِهِ

Muhammad ibn Ibrahim ibn Ishaq al-Taliqani - may God be pleased with him - narrated that Ahmad ibn Muhammad ibn Sa’eed al-Kufi quoted on the authority of Ali ibn Al-Hassan ibn Ali ibn Fadhdhal, on the authority of his father:

I asked Abal Hassan Al-Reza (s), “Why was the Prophet’s (s) nickname Abul Qasim?”

Al-Reza (s) said, “It was because he had a son named Qasim. That was why his nickname was Abul Qasim.”

I asked him, “O son of God’s Prophet (s)! Do you consider me worthy of more explanation?”

Al-Reza (s) said, “Yes. Don’t you know that God’s Prophet (s) said?, “Ali and I are the fathers of this nation.”

I said, “Yes, I do.”

Al-Reza (s) said, “Don’t you know that God’s Prophet (s) is the father of all the nation, and Ali is one of the members of this nation?”

I said, “Yes.”

Al-Reza (s) said, “Don’t you know that Ali (s) is the ‘Qasim or the one who divides up Paradise and Hell?”

I replied, “Yes.”

Al-Reza (s) said, “Then that is why the Prophet (s) is called Abul Qasim - as he is the father of the one who divides up Paradise and Hell.”

I asked, “What does this mean?”

Al-Reza (s) replied, “The kindness of the Prophet (s) for his nation is like that of a father for his children. The noblest member of his nation is Ali (s). And after him the kindness of Ali (s) for them is like the kindness of the Prophet (s), since he is the Prophet’s (s) Trustee, Successor and the Divine Leader after the Prophet (s). That is why the Prophet (s) said, “Ali and I are the two fathers of this nation.” God’s Prophet (s) climbed up the pulpit and said, “Whoever leaves behind a debt or a wife, I must repay his debt and pay for the living expenses of his wife. Whoever leaves behind some property, that property will belong to his inheritors.” That is why the Prophet (s) was superior to their fathers, mothers and even themselves. The same thing held true for the Commander of the Faithful (s) after God’s Prophet (s).

[Source: Uyun Al Akhbar Al Reza Vol. 2 Ch. 32 Hadees. 29]