The Book of Prayers
Ṣaḥīḥ Muslim 417
Say this
اللَّهُمَّ رَبَّنَا لَكَ الْحَمْدُ . وَإِذَا صَلَّى قَائِمًا فَصَلُّوا قِيَامًا وَإِذَا صَلَّى قَاعِدًا فَصَلُّوا قُعُودًا أَجْمَعُونَ
Allāhumma rabbanā laka al-ḥamdu . wa-idhā ṣallā qāʾiman faṣallūā qiyāman wa-idhā ṣallā qāʿidan faṣallūā quʿūdan ajmaʿūna
Transliteration generated from the Arabic vowel marks
The full narration
Abu Huraira reported Allah's Messenger (ﷺ) saying:The Imam is appointed to be followed. So recite takbir when he recites it, and bow down when he bows down and when he utters:" Allah listens to him who praises Him," say" O Allah, our Lord, for Thee be the praise." And when he prays, standing, you should pray standing. And when he prays sitting, all of you should pray sitting
Source
Ṣaḥīḥ Muslim · 417
Ṣaḥīḥ
- Recorded in Ṣaḥīḥ Muslim, whose narrations are accepted as authentic