What should one do if one forgets to mention a required tasbeeh during ruku’ and remembers just afterwards after rising up?
If someone, after raising his head from ruku‘, remembers that he did not say the supplications of ruku‘, he should not return back to the ruku‘ because the requirement for supplication of ruku‘ has been dropped by virtue of his rising it. If he returns back to the ruku‘ intentionally, this action would render his prayer invalid since he added an extra rukn (pillar, or obligatory act of the prayer) which is this second superfluous ruku‘. However, if he did it in ignorance or forgetfulness, his prayer will not be nullified, but in this case he must make the prostration of forgetfulness if he was praying individually or leading a congregation. This is because tasbeeh (subhaana Rabbi al-Azeem, How Perfect is my Lord, the Supreme, or another supplication of ruku‘) is wajib (required), and forgetting it makes the prostration of forgetfulness necessary. If he was praying behind an Imam, the he is released from the requirement of sujood for his forgetfulness (al-Mughni 1/679).