From Sahih Bukhari (Hadith 7103):
Narrated Abu Wail:Abu Musa and Abu 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 criticizable to us than your haste in this matter." `Ammar said, "Since you (both) became Muslims, I have never seen you doing a deed more criticizable 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)
Reference: Bukhari 7103