Musnad Ahmad 767:1

Waki’ narrated to us, Sufyan narrated to us, from Simak, from Suwayd ibn Qays, who said: I and Makhramah al-‘Abdi brought garments from Hajar. He said: The Messenger of Allah (peace and blessings of Allah be upon him) came to us and bargained with us for some trousers, and we had weighers who were weighing for a fee. He said to the weigher: "Weigh and give extra."