Musannaf Ibn Abi Shayba 36:52

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. As for the Messenger of Allah (peace be upon him), his uncle protected him. As for Abu Bakr, his people protected him. The others were seized and dressed in iron coats of mail, then they were melted [exposed to the heat] in the sun until the distress reached them to every extent. They gave them what they asked for. Then the people of each man among them came to them with leather mats containing water, and they threw them into them, then they carried them by their sides—except for Bilal. When evening came, Abu Jahl came and began to insult Sumayyah and speak obscenely, then he stabbed her and killed her; she was the first martyr to be martyred in Islam. As for Bilal, his soul became insignificant to him for the sake of Allah until they grew weary. They placed a rope around his neck, then ordered their children, and they ran with him between the two mountains of Makkah, while he kept saying: "One, One [Ahad, Ahad]."