Musannaf Ibn Abi Shayba 3:192

Abu Bakr narrated to us, he said: Ibn Numayr narrated to us, from Musa ibn Ubaydah, from Muhammad ibn Ka’b, that he heard him say: The minimum of prostration, when you place your head upon the earth, is that you say: "Subhana Rabbi al-A’la" (Glory be to my Lord, the Most High) three times.