Musnad Ahmad 30:243

Sufyan narrated to us, from Ibn ‘Ajlan, from Sa‘id, from Abu Hurayrah—if Allah wills. Then Sufyan said: That which we heard from him from Ibn ‘Ajlan, I do not know from whom. Sufyan was asked: About Thumamah ibn Uthal? He said: The Muslims had captured him; they took him. Whenever he [the Prophet] passed by him, he would say: "What do you have, O Thumamah?" He would say: "If you kill, you kill one who has blood [to be avenged], and if you show favor, you show favor to one who is grateful, and if you want wealth, you shall be given wealth." He said: Whenever he passed by him, he would say: "What do you have, O Thumamah?" He would say: "If you show favor, you show favor to one who is grateful, and if you kill, you kill one who has blood, and if you want wealth, you shall be given wealth." He said: Then it occurred to the Messenger of Allah, peace and blessings of Allah be upon him, and he released him. Allah, the Almighty and Majestic, cast [faith] into his heart. He said: So they took him to a well of the Ansar, and he washed himself, and he embraced Islam. He said: "O Muhammad, I entered the evening while your face was the most hateful of faces to me, your religion was the most hateful of religions to me, and your land was the most hateful of lands to me. And I entered the morning while your religion is the most beloved of religions to me, and your face is the most beloved of faces to me." [He vowed that] not a single grain from Al-Yamamah would reach Quraysh, until ‘Umar said: "By Allah, he was in my eyes smaller than a pig, and now he is in my eyes greater than a mountain." He let him go, so he went to Al-Yamamah and withheld [supplies] from them, until they cried out and became distressed. They wrote: "Will you command [us] to maintain ties of kinship?" He said: And he wrote to him. ‘Abdullah ibn Ahmad said: And I heard him say: From Sufyan, I heard Ibn ‘Ajlan, from Sa‘id, from Abu Hurayrah, that Thumamah ibn Uthal said to the Messenger of Allah, peace and blessings of Allah be upon him.