Musnad Ahmad 482:159

Yahya ibn Ishaq narrated to us, Ibn Lahi'ah narrated to us, from Yazid ibn Abi Habib, from Ibn Shamasa, from Uqbah ibn Amir, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The Muslim is the brother of the Muslim. It is not lawful for a Muslim man to conceal anything in his merchandise from his brother if he knows of it, [without] leaving it [disclosing it]."