Musnad Ahmad 834:17

'Abd al-Wahhab al-Khaffaf narrated to us, he said: Sa'id was asked about praying in the resting places of camels, so he informed us from Qatadah, from al-Hasan ibn Abi al-Hasan al-Basri, from 'Abdullah ibn Mughaffal, that the Messenger of Allah, peace and blessings of Allah be upon him, said: "If, meaning, the prayer overtakes you while you are in the resting places of camels, then do not pray. And if it overtakes you in the resting places of sheep, then pray if you wish."