Musannaf Ibn Abi Shayba 3:5376

Waki’ narrated to us, he said: Al-A’mash narrated to us, from Malik ibn al-Harith, from his father, he said: We were with Abu Musa in the post house (dar al-barid), and the time for prayer arrived. He led us in prayer upon dung, filth, and straw. We said: "Do you lead us in prayer here, while the open wilderness (al-barriyyah) is right beside you?" He said: "The open wilderness and here are the same."