Musnad Ahmad 30:3106

Waki’ narrated to us, he said: Ali ibn al-Mubarak narrated to us, from Yahya ibn Abi Kathir, from Abi Kathir al-Ghubari, from Abu Hurairah, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you sells a sheep — or a milch camel — do not leave the milk in its udder (to deceive the buyer)."