Musnad Ahmad 26:282

Yahya ibn Abi Bukayr narrated to us, Za'idah narrated to us, from 'Asim ibn Abi al-Najud, 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 a single person among them who did not agree with them on what they wanted, except for Bilal. For his soul became insignificant to him for the sake of Allah, and he became insignificant to his people, so they gave him to the children, and they began to take him around the mountain passes of Makkah, while he was saying: 'One, One.'"