Musnad Ahmad 1018:82

Aswad ibn Amir narrated to us, Abu Isra'il informed us, from al-Hakam, from Abu Sulayman, from Zayd ibn Arqam, who said: Ali adjured the people and said: "I adjure by Allah any man who heard the Prophet, may Allah bless him and grant him peace, say: 'O Allah, whoever I am his master (mawla), then Ali is his master. O Allah, befriend whoever befriends him, and be hostile to whoever is hostile to him.'" He said: So sixteen men stood up and testified.