Musannaf Ibn Abi Shayba 3:2484

Abu Bakr narrated to us, he said: Sahl ibn Yusuf narrated to us, from Humayd, from Bakr, he said: I wrote to Umar from Najran [saying] that they did not find a place cleaner or better than a church. He wrote back: "Sprinkle it with water and lotus leaves, and pray in it."