Salam,
Bismillah.
Bismillah.
Narrated Abu Huraira:
When the Prophet said, "Sami' a-l-lahu Liman hamida," (Allah heard those who sent praises to Him), he would say, "Rabbana wa-laka-l-hamd." On bowing and raising his head from it the Prophet used to say Takbir. He also used to say Takbir on rising after the two prostrations. (See Hadith No. 656).
Narrated Abu Huraira:
Allah's Apostle said, "When the Imam says, "Sami' a-l-lahu Liman hamida," you should say, "Allahumma Rabbana laka-l-hamd." And if the saying of any one of you coincides with that of the angels, all his past sins will be forgiven."
We'll do Hadith No. 763 tomorrow iA.
Narrated Rifa'a bin Rafi AzZuraqi:
One day we were praying behind the Prophet. When he raised his head from bowing, he said, "Sami'a-l-lahu Liman hamida." A man behind him said, "Rabbana walaka-l hamd hamdan Kathiran taiyiban mubarakan fihi" (O our Lord! All the praises are for You, many good and blessed praises). When the Prophet completed the prayer, he asked, "Who has said these words?" The man replied, "I." The Prophet said, "I saw over thirty angels competing to write it first." Prophet rose (from bowing) and stood straight till all the vertebrae of his spinal column came to a natural position.
So you may say Rabbanaa laka-l-hamd, or Rabbanaa walaka-l-hamd, or Allahumma rabbanaa laka-l-hamd, or Rabbana walaka-l hamd hamdan Kathiran taiyiban mubarakan fihi. Doesn't take long to day these absolutely beautiful words. So JUST DO IT! :D
Wassalam.