Here are a few narrations which show that Prophet Muhammad (ص) and the Imam's (ع) don't die like normal humans die and that they can be present in the world even after their death.

Hadith 1:

حدثنا معاوية بن حكيم عن الحسين بن على الوشا عن ابى الحسن الرضا عليه السلام قال قال لى بخراسان رايت رسول الله صلى الله عليه وآله هيهنا والتزمته

It has been narrated to us from Muawiya Bin Hakeem, from Al-Husayn Bin Ali Al-Washa, who has said:

Abu Al-Hassan Al-Reza (ع) said to me in Khurasan, "I saw the Messenger of Allah (ص) over here and offered my (Salam) to Him."

[Source: Basaair Al-Darajaat, Vol. 6, Chapter. 5, Hadees.1]

Hadith 2:

حدثنا محمد بن عيسى عن محمد بن سنان عن عمار بن مروان عن سماعة قال دخلت على ابى عبد الله عليه السلام وانا احدث نفسي فرعاني فقال مالك تحدث نفسك تشتهى ان ترى اباجعفر عليه السلام قلت نعم قال قم فادخل البيت فإذا هو أبو جعفر عليه السلام قال اتى قوم من الشيعة الحسن بن على عليه السلام بعد قتل امير المؤمنين عليه السلام فسألوه قال تعرفونامير المؤمنين عليه السلام إذا رأيتموه قالوا نعم قال فارفعوا الستر فرفعوه فإذا هم بامير المؤمنين عليه السلام لا ينكرونه وقال امير المؤمنين عليه السلام يموت من مات منا وليسبميت ويبقى من بقى منا حجة عليكم

It has been narrated to us from Muhammad Bin Isa, from Muhammad Bin Sinan, from Amaar Bin Marwaan, from Sama’at who said:

"I came to Abu Abdullah (ع) and I was talking to myself when He saw me. He said: ‘What is it with you? Would you like to see Abu Ja’far (ع)?"

I said, "Yes"

He said, "Get up". He made me to enter a room, and there was Abu Ja’far Imam Baqir (ع).

Imam Baqir (ع) said, "A group of Shiites came to Al-Hassan Bin Ali (ع) after the killing of Amir-ul-Momineen (ع). He asked them: 'Would you all recognize Amir-ul-Momineen (ع) if you were to see Him?"

They said, "Yes"

Al-Hasan (ع) raised the curtain, They all recognized that they were with Amir-ul-Momineen (ع), they could not deny it, and Amir-ul-Momineen (ع) said: 'When one of Us is dead, He is not dead, and when one of Us remains, the remaining One is a Proof on you all."

[Source: Basaair Al-Darajaat, Vol. 6, Chapter. 5, Hadees.4]

Hadith 3:

The last part of the above narration is also said by Imam Ali (as) in a sermon in Nahjul Balagha.

عَنْ خَاتَمِ النَّبِيِّينَ ص إِنَّهُ يَمُوتُ مَنْ مَاتَ مِنَّا وَ لَيْسَ بِمَيِّتٍ وَ يَبْلَى مَنْ بَلِيَ مِنَّا وَ لَيْسَ بِبَال

"From the Last of the Prophets (ص) who said, "He who dies from among us is not dead and he who decays from among us does not really decay."

[Source: Nahjul Balagha Sermon: 86/87]

Hadith 4:

حدثنى الحسين بن محمد بن عامر عن معلى بن محمد بن عبد الله عن بشير عن عثمان بن مروان عن سماعة بن مهران قال آنت عند ابى الحسن عليه السلام فاطلت الجلوس عنده فقال اتحب ان ترى ابا عبد الله عليه السلام فقال وددت والله فقال قم وادخل ذلك البيت فدخلت البيت فإذا هو أبو عبد الله صلوات الله عليه قاعد

It has been narrated to me Al-Husayn Bin Muhammad Bin Aamir, from Moala Bin Muhammad Bin Abdullah, from Bashir, from Usman Bin Marwan, from Sama’at Bin Mahran who said:

I was with Abu al-Hasan al-Kazim (ع). I extended my sitting with Him.

He said, "Would you love to see Abu Abdullah (ع)?"

I said, "I wish I could, by Allah (عز وجل)."

He said: "Get up and enter into that room."

I entered into the room, and there was Abu Abdullah (ع) sitting.

[Source: Basaair Al-Darajaat, Vol. 6, Chapter. 5, Hadees.8]