Sayyiduna Jarir (may Allah be pleased with him), whose kunyah was Abu Abdullah, narrates: He went early in the morning to the Kunasah (market) to buy an animal there, and a slave of his also went (with him) in the morning. He stood in a corner of the market, and the animals began to pass in front of him. A horse passed by him which he liked. He said to his slave: Go and buy that horse. So his slave went and offered its owner 300 dirhams for the horse, but its owner refused to sell it. He bargained with him, but its owner still refused to sell it. The slave said: Will you come with me to a companion of ours in the corner of the market? He said: I do not mind. So the two of them came to him, and the slave said to him (Jarir): I offered this man 300 dirhams for his horse, but he refused and said that it is worth more than that. The owner of the horse said: May Allah keep you righteous, he has told the truth; do you consider that (300 dirhams) a fair price? He said: No, your horse is better than that; sell it for 500, until he reached 700 or 800 dirhams. When the man had gone, he turned to his slave and said: Woe to you! You went to buy an animal for me; I liked a man's animal, so I sent you to buy it, but you brought along a man from the Muslims while he was saying: What do you think? What do you think? Yet I have given my pledge to the Messenger of Allah (peace and blessings be upon him) to wish well for every Muslim.
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 2382