Musnad Ahmad 907:64

Bahz narrated to us, Hammad bin Salamah narrated to us, from Simak, from Ja'far bin Abi Thawr bin Jabir bin Samurah, from his grandfather, that a man asked the Messenger of Allah (peace and blessings of Allah be upon him): "Should I perform ablution after eating sheep meat?" He said: "If you wish, you may do so, and if you wish, you may not." He said: "Should I perform ablution after eating camel meat?" He said: "Yes." He said: Then he turned away, then he returned and said: "O Messenger of Allah, may I pray in the sheep pens?" He said: "Yes." He said: "May I pray in the resting places of camels?" He said: "No."