Sunan an-Nasa'i 4066

It was narrated from Abu Burdah bin Abi Musa Al-Ash'ari, from his father:"That the Prophet [SAW] sent him to Yemen, then he sent Mu'adh bin Jabal after that. When he arrived he said: 'O people, I am the envoy of the Messenger of Allah [SAW] to you.' Abu Musa gave him a cushion to sit down, then a man was brought who had been a Jew, then he became a Muslim, then he reverted to Kufr. Mu'adh said: 'I will not sit down until he is killed; this is the decree of Allah and His Messenger,' (saying it) three times. When he was killed, he sat down