One day we were praying behind Allah's Messenger (sallallaahu 'alaihi wasallam). When he raised his head from bowing, he said, "Sami'a-l- lahuLiman 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 (sallallaahu 'alaihi wasallam) 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.

Sahih Al-Bukhari Vol. 1 : No. 764