Musnad Ahmad 30:2697

Yazid narrated to us, he said: Hisham—meaning Ibn Hassan—informed us, from Muhammad, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, who said: "If you do not find anything except 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."