Musannaf Ibn Abi Shayba 36:59

Yahya ibn Abi Bukayr narrated to us, he said: Za'idah narrated to us, from 'Asim, from Zirr, from 'Abdullah, who said: The first to manifest their Islam were seven: the Messenger of Allah (peace and blessings of Allah be upon him), Abu Bakr, 'Ammar, his mother Sumayyah, Suhayb, Bilal, and al-Miqdad. As for the Messenger of Allah (peace and blessings of Allah be upon him), Allah protected him through his uncle Abu Talib. As for Abu Bakr, Allah protected him through his people. As for the rest of them, the polytheists seized them, clothed them in coats of iron, and scorched them in the sun. There was not one of them who did not agree to what they wanted, except for Bilal. For he considered his own self insignificant for the sake of Allah, and he became insignificant to his people. So they gave him to the children, who began to parade him through the mountain paths of Makkah, while he was saying, "One, One (Ahad, Ahad)."