Musannaf Ibn Abi Shayba 28:201

'Abbad ibn al-'Awwam narrated to us, from Sa'id, from Qatadah, from Humayd ibn Hilal, that Mu'adh ibn 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 had been seeking his repentance for two months. Mu'adh said, "I will not sit until I strike his neck; Allah has decreed, and the Messenger of Allah, may Allah bless him and grant him peace, has decreed."