Musnad Ahmad 30:405

'Abd al-Wahid narrated to us, from 'Awf, from Khilas ibn 'Amr and Muhammad ibn Sirin, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever buys a milk-camel that has been tied up (to withhold its milk), or a sheep that has been tied up, and milks it, he has one of two choices: he has the option until he takes possession of it, or he returns it along with a vessel of food."