Musannaf Ibn Abi Shayba 13:858

Abu Bakr narrated to us, he said: Waki’ narrated to us, from ‘Ali ibn al-Mubarak, from Yahya ibn Abi Kathir, from Abi Kathir, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "When one of you sells a milch camel or a sheep, let him not withhold its milk (in its udder)."