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."
حَدَّثَنَا مُحَمَّدُ بْنُ بَشَّارٍ، حَدَّثَنِي حَمَّادُ بْنُ مَسْعَدَةَ، قَالَ حَدَّثَنَا قُرَّةُ بْنُ خَالِدٍ، عَنْ حُمَيْدِ بْنِ هِلاَلٍ، عَنْ أَبِي بُرْدَةَ بْنِ أَبِي مُوسَى الأَشْعَرِيِّ، عَنْ أَبِيهِ، أَنَّ النَّبِيَّ صلى الله عليه وسلم بَعَثَهُ إِلَى الْيَمَنِ ثُمَّ أَرْسَلَ مُعَاذَ بْنَ جَبَلٍ بَعْدَ ذَلِكَ فَلَمَّا قَدِمَ قَالَ أَيُّهَا النَّاسُ إِنِّي رَسُولُ رَسُولِ اللَّهِ إِلَيْكُمْ ‏.‏ فَأَلْقَى لَهُ أَبُو مُوسَى وِسَادَةً لِيَجْلِسَ عَلَيْهَا فَأُتِيَ بِرَجُلٍ كَانَ يَهُودِيًّا فَأَسْلَمَ ثُمَّ كَفَرَ فَقَالَ مُعَاذٌ لاَ أَجْلِسُ حَتَّى يُقْتَلَ قَضَاءُ اللَّهِ وَرَسُولِهِ ‏.‏ ثَلاَثَ مَرَّاتٍ ‏.‏ فَلَمَّا قُتِلَ قَعَدَ ‏.‏