Straightening up in prayer
Said as the back comes up out of the bow.
إِنَّمَا الإِمَامُ ـ أَوْ إِنَّمَا جُعِلَ الإِمَامُ ـ لِيُؤْتَمَّ بِهِ، فَإِذَا كَبَّرَ فَكَبِّرُوا، وَإِذَا رَكَعَ فَارْكَعُوا، وَإِذَا رَفَعَ فَارْفَعُوا، وَإِذَا قَالَ سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ. فَقُولُوا رَبَّنَا لَكَ الْحَمْدُ. وَإِذَا سَجَدَ فَاسْجُدُوا
Samiʿa-llāhu li-man ḥamidah. Rabbanā wa laka-l-ḥamd.
“Allah hears whoever praises Him. Our Lord, and to You is all praise.”
One half is said by the one praying and the other half answers it. The prayer is built as a conversation.
The full narration
Narrated Anas bin Malik:Allah's Messenger (ﷺ) fell from a horse and got injured so he led the prayer sitting and we also prayed sitting. When he completed the prayer he said, "The Imam is to be followed; if he says Takbir then say Takbir, bow if he bows; raise your heads when he raises his head, when he says, 'Sami`a l-lahu liman hamidah say, 'Rabbana laka l-hamd', and prostrate when he prostrates
Source
Ṣaḥīḥ al-Bukhārī · 733
- Agreed upon as authentic (al-Bukhārī / Muslim)