Sunan an-Nasa'i, Volume 2, Book 12, Hadith 1061 (DarusSalam) ⓘ
It was narrated that Abu Hurairah said: "When the Prophet raised his head from bowing, he said: 'Allahumma Rabbana wa lakal-hamd (O Allah, our Lord and to You be the praise).'"
أَخْبَرَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، قَالَ حَدَّثَنَا عَبْدُ الرَّزَّاقِ، قَالَ حَدَّثَنَا مَعْمَرٌ، عَنِ الزُّهْرِيِّ، عَنْ أَبِي سَلَمَةَ، عَنْ أَبِي هُرَيْرَةَ، قَالَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم إِذَا رَفَعَ رَأْسَهُ مِنَ الرُّكُوعِ قَالَ " اللَّهُمَّ رَبَّنَا وَلَكَ الْحَمْدُ " .
also referenced under: Hadith 1060 (Reference) & Book 12, Hadith 32 (In-Book)