Musannaf Ibn Abi Shayba 27:696

Jarir narrated to us, from Mansur, from Mujahid, who said: The first to manifest Islam were seven: the Messenger of Allah (peace be upon him), Abu Bakr, Bilal, Khabbab, Suhayb, Ammar, and Sumayyah, the mother of Ammar. He said: As for the Messenger of Allah (peace be upon him), his uncle protected him. As for Abu Bakr, his people protected him. As for the others, they were seized and dressed in iron coats of mail, then they were roasted in the sun until the distress reached them to every extent. They gave them whatever they asked for. Then the people of each man came to them with leather mats containing water, and they threw them into it, then they lifted them by the edges—except for Bilal. They placed a rope around his neck, then ordered their children to run with him between the two mountains of Makkah, while he kept saying, "One, One."