Musannaf Ibn Abi Shayba 3:5640

Abu Bakr narrated to us, he said: Isma'il ibn 'Ulayyah narrated to us, from Sa'id ibn Abi Sadaqah, he said: I said to Ibn Sirin, "If I sneeze during the prayer, what should I say?" He said: "Say, 'Praise be to Allah, Lord of the worlds.'"