Tasbih in ruku and sujood
Tasbih in ruku’ and sujood are one of the things the scholars have differed in opinion over, there are three opinions:
The first is that tasbih is sunnah, and not obligatory; the second opinion is that tasbih is obligatory, and if not read on purpose the prayer becomes invalid, if one forgets, the prayer is still valid; the third opinion is that tasbih is obligatory, if one leaves it intentionally, the prayer is invalid, if one forgets it is still valid and one must do a sajda after the prayer.
The majority opinion of the scholars is that it is a sunnah and it is the most correct opinion.
Al-Shawkani says in his comments on the hadith of Hudhayfah (He prayed along with the Prophet ﷺ and that he said when bowing, “Glory be to my mighty Lord”, and when prostrating himself, “Glory be to my most high Lord”; when he came to a verse which spoke of mercy he stopped and made supplication, and when he came to a verse which spoke of punishment he stopped and sought refuge in Allah SWA.) narrated by the five and Al tirmidhi authenticated it.
The hadith shows evidence on the legislation of the tasbih in the ruku’ (bowing) and sujood (prostrating), and Shafi’, Malik, Abu Hanifa and majority of the scholars have said it is sunnah, and not an obligation.
Ishaaq bin Rahawaih said: Tasbih is obligatory, if one leaves it on purpose his prayer is invalidated, and if he forgets it is not invalidated.
In the school of thought of Al Dhahiris’: It is obligatory, and he referred to Al-khattabi’s book named (Maa’lim us-sunan).
Ahmad said: Tasbih in the ruku’ and Sujood, the saying: (Sami’ Allahu liman hamidah) ‘Allah listens to those who praise Him, (Rabbana wa lakal-hamd) ‘O our Lord! And to You is the praise, and the remembrance between the two (sajdah) prostrations, and all of the takbeers (saying Allahu Akbar) are all obligatory, and if you leave one of them on purpose your prayer is invalidated, and if you forget then it is not invalidated but you must do a sajdat us- sahw (prostration of forgetfulness). This is the correct from him. There is also a narration of him saying it is sunnah like the majority.
Ibn Khuzaimah was also reported that he supported the opinion that sees Tasbih in ruku’ and sujood obligatory acts of salah.
They brought evidence of the Prophet ﷺ (Pray like you see me praying) and the saying of Allah SWA (and make tasbih- praise Him) and there is no obligation in other than the prayer so it must be there, and by using the analogy of reading.
However the majority brought the evidence of the person whose prayer was incorrect, for the Prophet ﷺ taught him the obligations of the prayer, and he did not teach him these remembrances, even though he taught him the Takbeerat al ihram and reading. If these tasbihs had been obligatory he would have taught him, because delaying the statement until time of need is not permissible, so having those cases ignored rendered them to be unnecessary and can only come under desirable actions but can not be obligatory.