Abdel Ghaffar ibn al-Qassim said: While Ali bin al-Hussein was leaving the mosque, a man cussed him out. The people got angry, but Ali bin al-Hussein said: "Leave him alone" Then he went to him and said: "What has been concealed from you about me is even a lot more than that. Do you need anything that I may help you with?" The man felt shy and ashamed of himself. Ali gave him a black cloak that he was wearing, and he gave him a thousand dirhams. The man used to say after this incident: I bear witness that you descend from the offspring of Messengers.
Reference: Thadheeb al-Kamal (13/246).