Is saying "Samia Allahu liman hamidah" (Allah hears the one who praises Him) and "Rabbana lakal Hamd" (Our Lord, to You be all praise) in prayer obligatory or a Sunnah, and does the congregant say "Samia Allahu liman hamidah"?

Scholars differed on the ruling concerning the utterance of "Sami'a Allahu liman hamidah" (Allah hears those who praise Him) and "Rabbana wa lakal hamd" (Our Lord, to You be all praise) in prayer, holding two views: The majority consider it Sunnah (definition) (recommended), while the Hanbalis consider it Wajib (definition) (obligatory). The Hanbalis based their view on the hadith (definition), "Pray as you have seen me praying," and the commands of the Prophet, peace and blessings be upon him, and because uttering "Sami'a Allahu liman hamidah" and "Rabbana wa lakal hamd" is a sign of transition [between prayer postures]. As for the hadith of the one who prayed incorrectly, it mentioned teaching that, but the Prophet, peace and blessings be upon him, did not teach him all the obligatory acts. The individual praying alone and the Imam combine both "Sami'a Allahu liman hamidah" and "Rabbana wa lakal hamd", whereas the follower (Ma'mum) restricts himself to "Rabbana wa lakal hamd".