What is the ruling on an individual saying "Our Lord, and to You be all praise"? And what should someone do who has been praying his whole life without saying it?

The saying "Rabbana wa lakal-hamd" (O our Lord, and to You be all praise) upon rising from bowing is considered obligatory by some scholars and sunnah by others. For someone who omitted it out of ignorance, there is nothing upon them, and no making up is required, as Sheikh Al-Uthaymeen stated that whoever omits an obligatory act due to ignorance, their prayer is valid, and they are not required to perform sujud as-sahw (prostration of forgetfulness). Therefore, there is nothing upon the questioner for having omitted the tahmid (saying "Rabbana wa lakal-hamd") in the past due to ignorance. In the future, they should strive to observe the pillars and obligatory acts of prayer and gain a deeper understanding of their worship.