Abd Allah is the youngest son of Imam al-Husayn (a). His mother was Rabab bt. Imru' al-Qays.

There is no consensus on the exact date of his birth. However, most historical references have mentioned that he was newly born when he was martyred on the Day of Ashura (61/681). He is buried in Karbala next to the tomb of Imam al-Husayn (a).

Both Shi'a and Sunni early sources have mentioned that the name of this infant was Abd Allah; however, in later Shi'a sources, he is known as Ali al-Asgar.

The books Maqtal al-Husayn by Khwarizmi and Manaqib Al Abi Talib by Ibn Shahrashub are two of the above-mentioned later Shi'a sources which have mentioned the youngest son of Imam al-Husayn (a) as Ali. Later sources followed them in calling this infant Ali al-Asghar (a) (especially in Persian sources).