Musannaf Ibn Abi Shayba 35:9

Yazid ibn Harun narrated to us, Hisham narrated to us, from Muhammad ibn Sirin, from Abu Hurayrah, from the Prophet (may Allah bless him and grant him peace), who 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."