Musnad Ahmad 30:2424

Yahya narrated to us, from ‘Awf, who said: Khallas narrated to us, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "The likeness of the one who returns to his gift is the likeness of the dog; when it is full, it vomits, then it returns to its vomit."