Musnad Ahmad 26:246

Khalaf ibn al-Walid narrated to us, Isra'il narrated to us, from Abu Ishaq, from 'Amr ibn Maymun, from 'Abdullah, who said: Sa'd ibn Mu'adh set out to perform 'Umrah, and he stayed with Umayyah ibn Khalaf ibn Safwan. Whenever Umayyah would set out to the Levant and pass by Medina, he would stay with Sa'd. He mentioned the hadith, except that he said: He returned to Umm Safwan and said, "Do you not know what my Yathribi brother said?" She said, "And what did he say?" He said, "He claims that he heard Muhammad claiming that he is my killer." She said, "By Allah, Muhammad does not lie." Then, when they went out to Badr, and he drove him [to the battle]...