حدثنا محمد بن إبراهيم بن إسحاق الطالقاني رضي الله عنه قال: حدثنا أحمد محمد بن سعيد الكوفي قال: حدثنا علي بن الحسن بن علي بن فضال عن أبيه قال: سألت أبا الحسن عليه السلام، فقلت له: لم كني النبي (ص) بأبي القاسم؟ فقال لانه كان له ابن يقال له قاسم فكني به قال: فقلت له يا بن رسول الله فهل تراني أهلا للزيادة فقال: نعم أما علمت إن رسول الله (ص) قال: أنا وعلي أبوا هذه الامة؟ قلت: بلى قال: أما علمت إن رسول الله (ص) أب لجميع أمته وعلي عليه السلام منهم قلت: بلى قال: أما علمت أن عليا عليه السلام قاسم الجنة والنار قلت: بلى قال فقيل له أبو القاسم لانه أبو قسيم الجنة والنار فقلت له: ومعنى ذلك؟ قال: إن شفقة النبي (ص) على أمته شفقة الاباء على الاولاد، وأفضل أمته علي عليه السلام ومن بعده شفقة علي عليه السلام عليهم كشفقته (ص) لانه وصيه وخليفته والامام بعده فلذلك قال: أنا وعلي أبوا هذه الامة وصعد النبي (ص) المنبر فقال من ترك دينا أو ضياعا فعلي والي ومن ترك مالا فلورثته فصار بذلك أولى بهم من آبائهم وأمهاتهم وأولى بهم بانفسهم، وكذلك أمير المؤمنين عليه السلام بعده جرى ذلك له مثل ما جرى لرسول الله صلى الله عليه وآله

The narrator says, I asked Abu al-Hasan al-Ridha (عليه السلام), 

"Why was the Prophet (صلى الله عليه وآله) given the kunya (honorific title) of Abu al-Qasim?"

The Imam replied, "Because he had a son named Qasim, so he was given the kunya referring to him."

I (the narrator) then said, "O son of the Messenger of Allah! Do you find me worthy of further [explanation]?"

The Imam replied, "Yes. Do you not know that the Messenger of Allah (صلى الله عليه وآله) said - I and Ali (عليه السلام) are the two fathers of this nation?"

I (the narrator) said, "Indeed."

The Imam said, "Do you not know that the Messenger of Allah (صلى الله عليه وآله) is a father to his entire nation, and Ali (عليه السلام) is also included in the nation?"

I (the narrator) said, "Indeed."

The Imam said, "Do you not know that Ali is the Divider (Qasim) of Paradise and Hellfire?"

I said, "Indeed."

He said, "Therefore, the Prophet (S) was called Abu al-Qasim because he is the father of the Divider (Qasim) of Paradise and Hellfire."

I (the narrator) said, "And what is the meaning of that?"

The Imam replied: "The compassion of the Messenger for his nation is like the compassion of fathers for their children. The most excellent of his nation is Ali (عليه السلام), and after him, Ali’s (عليه السلام) compassion for them is like the Prophet's compassion, because he is his successor, his caliph, and the Imam after him.

That is why the Prophet (S) said - "I and Ali (عليه السلام) are the two fathers of this nation." 

The Prophet (S) ascended the pulpit and said - "Whoever leaves behind a debt or a loss (dependents), it is upon me and comes to me; and whoever leaves behind wealth, it is for his inheritors."

By this, he became more deserving to them (closer to them) than their own fathers and mothers, and more entitled to them than they are to themselves. Likewise, the same applied to the Amir al-Momimin (عليه السلام) after him, just as it applied to the Messenger of Allah (صلى الله عليه وآله)."

[Source: Uyun Akhbar al-Ridha, Ch. 32, Hadith. 29]