Musnad Ahmad 30:3475

Yazid narrated to us, Hammad ibn Salamah informed us, from Ali ibn Zayd, from Aws ibn Khalid, from Abu Hurayrah, from the Prophet (peace and blessings of Allah be upon him), who said: "The likeness of the one who hears wisdom, then does not report from its owner except the worst of what he hears, is like a man who came to a shepherd and said: 'Slaughter a sheep for me from your flock.' He said: 'Choose.' So he took the ear of the sheepdog."