Musannaf Ibn Abi Shayba 36:78

Aswad ibn Amir narrated to us, he said: Hammad ibn Salamah narrated to us, from Ali ibn Zayd, from al-Hasan, from Suraqah ibn Malik al-Mudliji, that he told them that Quraysh had set a bounty of forty ounces for [the capture of] the Messenger of Allah (may Allah bless him and grant him peace) and Abu Bakr. He said: While I was sitting, a man came to me and said, "The two men whom Quraysh have set what they have set for are near you in such and such a place." So I went to my horse, and it was in the low ground, and it shied away from me. Then I took my spear, and he said: So I mounted it, and I began to drag the spear for fear that the people of the water [the tribe] might share in [the bounty for] them with me. He said: When I saw them, Abu Bakr said, "This is a seeker seeking us." The Prophet (may Allah bless him and grant him peace) turned to me and said, "O Allah, suffice us against him with whatever You will." He said: He said, "My horse stumbled while I was on a level piece of land, and I fell onto a rock, and it [the horse] overturned. I said, 'Pray to the One who did to my horse what I see, that He may release it,' and I made a covenant with Him that I would not disobey Him." He said: So he prayed for it, and the horse was released. The Messenger of Allah (may Allah bless him and grant him peace) said, "Will you gift it to me?" I said, "Yes." He said, "Then [go] this way." He said: So he blinded the people from us, and the Messenger of Allah (may Allah bless him and grant him peace) took the coastal road along the sea. He said: I was at the beginning of the day a seeker of them, and at the end of the day a guard for them. He said to me, "When we are settled in Medina, if you see fit to come to us, then come to us." He said: When he arrived in Medina and prevailed over the people of Badr and Uhud, and the people and those around them embraced Islam, Suraqah said: "It reached me that he intended to send Khalid ibn al-Walid to Banu Mudlij." He said: So I came to him and said, "I adjure you by the favor." The people said, "Hold!" The Messenger of Allah (may Allah bless him and grant him peace) said, "Leave him." The Messenger of Allah (may Allah bless him and grant him peace) said, "What do you want?" I said, "It reached me that you intend to send Khalid ibn al-Walid to my people, and I would like for you to make a peace treaty with them. If their people embrace Islam, they will embrace Islam with them, and if they do not embrace Islam, the hearts of their people will not be hardened against them." The Messenger of Allah (may Allah bless him and grant him peace) took the hand of Khalid ibn al-Walid and said to him, "Go with him and do what he wants." So he went to Banu Mudlij, and they took a covenant from them that they would not aid [anyone] against the Messenger of Allah (may Allah bless him and grant him peace), and that if Quraysh embraced Islam, they would embrace Islam with them. Then Allah revealed: "They wish you would disbelieve," until He reached: "Except those who join a people between whom and yourselves there is a treaty, or who come to you, their hearts constrained from fighting you or fighting their own people. And if Allah had willed, He could have given them power over you, and they would have fought you." Al-Hasan said: Those whose hearts were constrained were Banu Mudlij, and whoever joined Banu Mudlij from others was in the same [status] as their covenant.