Musannaf Ibn Abi Shayba 19:916

Abu Bakr narrated to us, he said: 'Abbad bin al-'Awwam narrated to us, from Sa'id, from Qatadah, from Humayd bin Hilal, that Mu'adh bin Jabal came to Abu Musa, and with him was a Jewish man. He said: "What is this?" He said: "This is a Jew who became Muslim, then apostatized, and Abu Musa has been seeking his repentance for two months." He said: So Mu'adh said: "I will not sit until I strike his neck; Allah has decreed and His Messenger has decreed."