Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) said: When the Messenger of Allah (peace and blessings be upon him) would raise his head from bowing (ruku‘), he would say: «سَمِعَاللّٰهُلِمَنْحَمِدَهُ،رَبَّنَالَكَالْحَمْدُمِلْءَالسَّمَاوَاتِوَالْاَرْضِوَمِلْءَمَابَيْنَهُمَاوَمِلْءَمَاشِئْتَمِنْشَيْيءٍبَعْدُ». Imam Darimi (may Allah have mercy on him) was asked: Do you say this? He replied: No. He was asked: Do you say this in the obligatory prayer? He replied: Almost, and he explained that saying everything (that is narrated) is correct.
Hadith Referenceسنن دارمي / من كتاب الصللاة / 1351
(Explanation of Ahadith 1348 to 1351) From these narrations, it is understood that for the imam and the individual praying alone (munfarid) to say «سَمِعَاللّٰهُلِمَنْحَمِدَه» while rising from bowing (ruku‘), and for all followers (muqtadis) to say «رَبَّنَالَكَالْحَمْدُ» or «اَللّٰهُمَّرَبَّنَاوَلَكَالْحَمْدُ», is among the obligations (wajibat) of prayer. This is because of the Prophetic command: «فقولوا: رَبَّنَا». After this, to say «حَمْدًاكَثِيْرًاطَيِّبًامُبَارَكًافِيْهِ» or «رَبَّنَاوَلَكَالْحَمْدُمِلْءَالسَّمَاوَاتِ» as mentioned in the hadith, all these remembrances (adhkar) are established from authentic ahadith; therefore, reciting them is also Sunnah. To proceed directly to prostration (sajdah) after rising from bowing, without reciting anything, is contrary to the Sunnah of the Messenger (sallallahu alayhi wa sallam).