Musannaf Ibn Abi Shayba 27:435

Sharīk narrated to us, from Ḥanash ibn al-Ḥārith, from Rabāḥ ibn al-Ḥārith, who said: While ‘Alī was sitting in the Raḥbah, a man came upon whom were the traces of travel. He said, "Peace be upon you, O my master (Mawlā)." He ( ‘Alī) said, "Who is this?" They said, "This is Abū Ayyūb al-Anṣārī." He said, "I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Whoever I am his master (Mawlā), then ‘Alī is his master (Mawlā).'"