Musannaf Ibn Abi Shayba 3:1503

Yazid ibn Harun narrated to us, he said: Hisham informed us, from Muhammad, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "If you do not find anything except the resting places of sheep and the kneeling places of camels, then pray in the resting places of sheep, and do not pray in the kneeling places of camels."