Is it permissible to combine the ذكرs (words of remembrance) for rukoo’ (bowing) that were authentically reported from the Prophet (peace and blessings of Allah be upon him) in the prayer, or should one limit oneself to only one ذكر?

It is permissible to limit oneself to glorification (tasbih) in bowing (ruku') and prostration (sujud), and it is legislated to combine the transmitted invocations (adhkar) and supplications (du'a). If the worshipper is praying alone, whether in a fard (obligatory) or nafl (supererogatory) prayer, he may prolong it as much as he wishes, as this has been narrated from the Prophet, peace and blessings be upon him. As for the Imam, he should not prolong the prayer in a way that causes hardship to the worshippers, although he may combine some invocations and supplications.