Ibn Nafi' narrates from Ibn Matar: I came out of the mosque, and a man called out to me from behind: "Raise your lower garment, for it keeps your clothes cleaner and is more fitting for you, and cut the hair of your head if you are a Muslim." I walked behind him and said: Who is this? A man replied: This was the Commander of the Believers, Sayyiduna Ali (may Allah be pleased with him). The narrator said: Then he came to the house of Furat, which is the market of cotton cloth (karabis). He said: "O Shaykh! Sell me a shirt for three dirhams." When the man recognised him, he did not sell him anything. When he came to another, he too, after recognising him, sold him nothing. Then he came to a young lad, who sold him a shirt for three dirhams. He wore it, (reaching) from his wrists down to his ankles. The lad's father came to the cloth seller, and he was told: O so-and-so! Your son sold a shirt to the Commander of the Believers today for three dirhams. The narrator said: The lad's father said: Why did you not take two dirhams? Then the lad's father took a dirham, brought it to the Commander of the Believers and said: O Commander of the Believers! Take your dirham. Sayyiduna Ali (may Allah be pleased with him) asked: "What is this dirham?" He said that the price of the shirt was two dirhams. He said: "I bought it with my consent, and he received the dirhams from me with his consent."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20293