Musannaf Ibn Abi Shaybah

It is narrated from Hazrat Abdullah that the first seven people who openly declared their Islam were: the Messenger of Allah (peace and blessings be upon him), Hazrat Abu Bakr, Hazrat Ammar, his mother Hazrat Sumayyah, Hazrat Bilal, Hazrat Suhaib, and Hazrat Miqdad. So Allah Almighty protected the Messenger of Allah (peace and blessings be upon him) from the polytheists through his uncle Abu Talib, and Allah Almighty protected Hazrat Abu Bakr from the polytheists through his own people. As for the remaining individuals, the polytheists captured them, made them wear iron shirts, and left them to burn in the sun. Then, except for Hazrat Bilal, there was none among them who did not comply with the wishes of the polytheists. Hazrat Bilal considered his life insignificant for the sake of Allah, and he was also considered insignificant by his people. So the polytheists handed Hazrat Bilal over to the children, and they began to drag him through the valleys, while Hazrat Bilal kept saying: "Ahad, Ahad" (One, One).