Musnad Abu Ya'la al-Mawsili 117:109

Abu Bakr ibn Abi Shaybah narrated to us, Muhammad ibn al-Hasan narrated to us, Abu Jami' al-Hujaymi narrated to us, from Thabit, from Anas: That the Messenger of Allah (peace and blessings of Allah be upon him) gave a young boy to Ali and Fatimah and said, "Treat him well, for I have seen him praying."