Musannaf Ibn Abi Shayba 27:695

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 (may Allah bless him and grant him peace), Abu Bakr, 'Ammar, his mother Sumayyah, Bilal, and al-Miqdad. As for the Messenger of Allah (may Allah bless him and grant him peace), 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 and clothed them in coats of iron and melted them in the sun. There was not one of them who did not come to them [agreeing] to what they wanted, except for Bilal. For he considered his soul insignificant for the sake of Allah, and he became insignificant to his people. So they took him and gave him to the children, and they began to walk him around in the mountain paths of Makkah, while he was saying: "Ahad, Ahad (One, One)."