Sahih Bukhari Volume 9, Book 92, Hadith 7105

حَدَّثَنَا بَدَلُ بْنُ الْمُحَبَّرِ، حَدَّثَنَا شُعْبَةُ، أَخْبَرَنِي عَمْرٌو، سَمِعْتُ أَبَا وَائِلٍ، يَقُولُ دَخَلَ أَبُو مُوسَى وَأَبُو مَسْعُودٍ عَلَى عَمَّارٍ حَيْثُ بَعَثَهُ عَلِيٌّ إِلَى أَهْلِ الْكُوفَةِ يَسْتَنْفِرُهُمْ فَقَالاَ مَا رَأَيْنَاكَ أَتَيْتَ أَمْرًا أَكْرَهَ عِنْدَنَا مِنْ إِسْرَاعِكَ فِي هَذَا الأَمْرِ مُنْذُ أَسْلَمْتَ‏.‏ فَقَالَ عَمَّارٌ مَا رَأَيْتُ مِنْكُمَا مُنْذُ أَسْلَمْتُمَا أَمْرًا أَكْرَهَ عِنْدِي مِنْ إِبْطَائِكُمَا عَنْ هَذَا الأَمْرِ‏.‏ وَكَسَاهُمَا حُلَّةً حُلَّةً، ثُمَّ رَاحُوا إِلَى الْمَسْجِدِ‏.‏

Narrated By Abu Wail : Abu Musa and Abi Mas'ud went to 'Ammar when 'Ali had sent him to Kufa to exhort them to fight (on 'Ali's side). They said to him, "Since you have become a Muslim, we have never seen you doing a deed more criticisable to us than your haste in this matter." 'Ammar said, "Since you (both) became Muslims, I have never seen you doing a deed more criticisable to me than your keeping away from this matter." Then Abu Mas'ud provided 'Ammar and Abu Musa with two-piece outfits to wear, and one of them went to the mosque (of Kufa).