Musnad Ahmad 30:1518

Hassan and ‘Affan narrated to us, they said: Hammad ibn Salamah narrated to us, from ‘Ali ibn Zayd, from Aws ibn Khalid, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The likeness of the one who sits and hears wisdom, then does not narrate from his companion except the worst of what he heard, is like a man who came to a shepherd and said: 'O shepherd, slaughter for me a sheep from your flock.' He said: 'Go and take the ear of the best of them.' So he went and took the ear of the sheepdog."