Musnad Ahmad 1060:66

Yahya ibn Adam narrated to us, Hanash ibn al-Harith ibn Laqit al-Nakha'i al-Ashja'i narrated to us, from Riyah ibn al-Harith, who said: A group came to Ali at al-Rahba and said: "Peace be upon you, our master (mawla)." He said: "How can I be your master when you are an Arab people?" They said: "We heard the Messenger of Allah, peace and blessings of Allah be upon him, on the day of Ghadir Khumm say: 'Whoever I am his master, then this [Ali] is his master.'" Riyah said: When they departed, I followed them and asked: "Who are these people?" They said: "A group of the Ansar, among them Abu Ayyub al-Ansari."