Congregational Prayer and Its Virtue
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 57
وَ رَوَى أَبُو أُسَامَةَ: أَنَّهُ سَأَلَهُ عَنْ رَجُلٍ اِنْتَهَى إِلَى اَلْإِمَامِ وَ هُوَ رَاكِعٌ، قَالَ «إِذَا كَبَّرَ وَ أَقَامَ صُلْبَهُ ثُمَّ رَكَعَ فَقَدْ أَدْرَكَ.
Abu Usamah narrated that he asked about a man who reaches the Imam while he is in the bowing position (ruku'). Imam عليه السلام said: "If he pronounces the takbir and straightens his back (stands up properly) and then bows, he has caught the rak'ah."
- Ṣaḥīḥ al-Faqīh