Jami` at-Tirmidhi 262

Hudhaifah narrated that :he performed Salat with the Prophet, and that while he was bowing he would say: (Subhana Rabbiyal Azim); "Glorious is my Lord the Magnificent" and while prostrating: (Subhana Rabbiyal A'la) 'Glorious is my Lord the Most High.' And he would not recite an Ayah mentioning mercy, except that he would stop and ask (for mercy), and he would not recite an Ayah mentioning punishment, except that he would stop and seek refuge (with Allah from it)