Sahih Bukhari, Chapter: Characteristics of Prayer

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.