Musnad Ahmad 341:2

Muhammad ibn Ja‘far narrated to us, he said: Shu‘bah narrated to us, from Mansur, who said: I heard Mujahid narrating from Abu ‘Ayyash al-Zuraqi—Shu‘bah said: He wrote it to me, and I read it to him, and I heard him narrating it, but I memorized it from the writing—that the Prophet, peace and blessings of Allah be upon him, was in the ranks of the enemy at ‘Usfan, and over the polytheists was Khalid ibn al-Walid. The Prophet, peace and blessings of Allah be upon him, prayed the Dhuhr prayer with them. Then the polytheists said: "They have a prayer after this one that is dearer to them than their children and their wealth." He said: "So the Messenger of Allah, peace and blessings of Allah be upon him, prayed the ‘Asr prayer with them, and he arranged them in two rows behind him. He said: The Messenger of Allah, peace and blessings of Allah be upon him, bowed with them all together. When they raised their heads, the row that was next to him prostrated, while the others remained standing. When they raised their heads, the rear row prostrated after their bowing with the Messenger of Allah, peace and blessings of Allah be upon him." He said: "Then the front row moved back and the rear row moved forward, and each one of them stood in the place of his companion. Then the Messenger of Allah, peace and blessings of Allah be upon him, bowed with them all together. When they raised their heads from the bowing, the row that was next to him prostrated, while the others remained standing." Then the Prophet, peace and blessings of Allah be upon him, performed the taslim upon them.