Sunan Ibn Majah — The Book of the Sunnah — Hadith No: 150
Translation
It was narrated that 'Abdullah bin Mas'ud said:"The first people to declare their Islam publicly were seven: The Messenger of Allah, Abu Bakr, 'Ammar and his mother Sumayyah, Suhaib, Bilal and Miqdad. With regard to the Messenger of Allah, Allah protected him through his paternal uncle Abu Talib. With regard to Abu Bakr, Allah protected him through his people. As for the rest, the idolators seized them and made them wear coats of chain-mail and exposed them to the intense heat of the sun. There was none of them who did not do what they wanted them to do, except for Bilal. He did not care what happened to him for the sake of Allah, and his people did not care what happened to him. Then they gave him to the children who took him around in the streets of Makkah while he was saying, 'Ahad, Ahad (One, One)