Musnad Ahmad 30:3235

Muhammad ibn Ja‘far narrated to us, he said: Hisham and Yazid narrated to us, they said: Hisham informed us, from Muhammad, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "If you find nothing but the resting places of sheep and the watering places of camels, then pray in the resting places of sheep, and do not pray in the watering places of camels."