Musnad Ahmad 455:15

Yazid ibn Harun narrated to us, Hammad ibn Salamah informed us, from Ya'la ibn 'Ata, from Yazid ibn Talq, from 'Abd al-Rahman ibn al-Baylamani, from 'Amr ibn 'Abasah al-Sulami, who said: I said: "O Messenger of Allah, who is with you in this matter?" He said: "A free man and a slave." And with him were Abu Bakr and Bilal. Then he said to him: "Return to your people until Allah, the Almighty and Majestic, establishes [the position of] His Messenger." 'Amr ibn 'Abasah used to say: "I have seen myself, and I was the fourth of Islam."