Congregational Prayer and Its Virtue
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 57
وَ رَوَى مُعَاوِيَةُ بْنُ شُرَيْحٍ عَنْ أَبِي عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ أَنَّهُ قَالَ: «إِذَا جَاءَ اَلرَّجُلُ مُبَادِراً وَ اَلْإِمَامُ رَاكِعٌ أَجْزَأَتْهُ تَكْبِيرَةٌ وَاحِدَةٌ لِدُخُولِهِ فِي اَلصَّلاَةِ وَ اَلرُّكُوعِ.
Mu'awiyah ibn Sharih reported from Imam Jafar ibn Muhammad Al-Sadiq عليه السلام that He said: "If a person comes quickly while the Imam is in ruku' (bowing), one takbir (saying 'Allahu Akbar') suffices for him to enter the prayer and perform the ruku'."
- Ṣaḥīḥ al-Faqīh