Musannaf Ibn Abi Shayba 28:631

Yazid ibn Harun narrated to us, he said: Muslim ibn Sa'id informed us, he said: Khubayb ibn Abd al-Rahman ibn Khubayb narrated to us, from his father, from his grandfather, who said: The Messenger of Allah (peace be upon him) went out intending a direction, and I came to him, I and a man from my people, and we said: "If our people witness a battle, we do not witness it with them." He said: "Have you both accepted Islam?" We said: "No." He said: "For we do not seek the help of the polytheists against the polytheists." He said: So we accepted Islam and witnessed [the battle] with him.