Musannaf Ibn Abi Shayba 3:5631

Abu Bakr narrated to us, he said: Hushaym narrated to us, from Mughirah, from Ibrahim, he said: Abu Hurayrah said: If one of you doubts about moisture while he is in prayer, let him place his hand upon the pebbles and wipe one of them with the other, and let him proceed in his prayer.