I say, and with God's help: If one forgets the bowing (ruku) and remembers it during the prostration (sujud), he should return to the bowing even if he has already prostrated. He should bow and then repeat the prostration, and he should perform the prostration of forgetfulness at the end of the prayer. However, if he forgets the bowing and does not remember it until after the prayer has ended, he must repeat the entire prayer; because omitting a mandatory act of prayer invalidates it. And God knows best.