Musnad Ahmad 30:581

'Abd al-Razzaq narrated to us, Ma'mar informed us, from Yahya ibn Abi Kathir, Abu Kathir informed me that he heard Abu Hurayrah saying: The Prophet, peace and blessings of Allah be upon him, said: "If one of you sells a sheep or a milch camel, let him not withhold its milk (in the udder)."