Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him) narrates that a man accepted Islam and then became a Jew. Mu'adh (may Allah be pleased with him) came to him while he was with Abu Musa (may Allah be pleased with him), and he asked (Abu Musa, may Allah be pleased with him): "What is this?" Abu Musa (may Allah be pleased with him) said: "This man accepted Islam and then became a Jew." Mu'adh (may Allah be pleased with him) said: "I will not sit until you kill him; this is the judgement of Allah and His Messenger."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17203