Musannaf Ibn Abi Shayba 26:551

Waki’ narrated to us, he said: Al-Rabi’ ibn Abi Salih al-Aslami narrated to us, from a man he named, that a man from the people of al-Sawad, who was called Hashi, came to Ali to form a client-pact (wala’) with him, but he refused to form a client-pact with him. He said: So he came to al-Abbas, or the son of al-Abbas, and he formed a client-pact with him.