Musnad Ahmad 150:1

Waki’ narrated to us, Sufyan narrated to us, from ‘Ata’ ibn al-Sa’ib, who said: I came to Umm Kulthum, daughter of ‘Ali, with something from the charity (sadaqah), and she rejected it and said: The freed slave of the Prophet, may Allah bless him and grant him peace, who is called Mihran, narrated to me that the Messenger of Allah, may Allah bless him and grant him peace, said: "Indeed, we, the family of Muhammad, charity is not lawful for us." And the freed slave of a people is one of them.