Musnad Ahmad 5:526

'Affan narrated to us, Abu 'Awanah narrated to us, Husayn narrated to us, Sa'd ibn 'Ubaydah narrated to me, from Abu 'Abd al-Rahman al-Sulami, from 'Ali, who said: The Messenger of Allah (peace and blessings of Allah be upon him) sent me, al-Zubayr, and Abu Marthad—and all of us were horsemen—and he said: "Go until you reach Rawdat Khakh"—that is how Abu 'Awanah said it—"for there is a woman there with a letter from Hatib ibn Abi Balta'ah to the polytheists." And he mentioned the rest of the hadith in full.