Musnad Ahmad 32:300

Yazid narrated to us, Hammad ibn Salamah narrated to us, from Thabit al-Bunani, from Anas ibn Malik, that Safiyyah fell into the share of Dihyah al-Kalbi. It was said: "O Messenger of Allah, a beautiful slave girl has fallen into the share of Dihyah." So the Messenger of Allah (peace and blessings of Allah be upon him) purchased her for seven heads [of livestock/slaves], and he placed her with Umm Sulaym so that she might prepare herself and complete her waiting period—according to what Hammad knows. The people said: "By Allah, we do not know whether the Messenger of Allah (peace and blessings of Allah be upon him) married her or took her as a concubine." When he carried her, he veiled her and had her ride behind him, so the people knew that he had married her. When he drew near to Medina, the people spurred their mounts, and the Messenger of Allah (peace and blessings of Allah be upon him) spurred his mount, and that is what they used to do. The she-camel stumbled, and the Messenger of Allah (peace and blessings of Allah be upon him) fell, and she fell with him. The wives of the Prophet (peace and blessings of Allah be upon him) were watching, and they said: "May Allah distance the Jewess," and they said this and that about her. The Messenger of Allah (peace and blessings of Allah be upon him) stood up, veiled her, and had her ride behind him.