وَ رَوَى الْحَلَبِيُّ عَنْ أَبِي عَبْدِ اللَّهِ ع أَنَّهُ قَالَ:
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إِذَا أَدْرَكْتَ الْإِمَامَ وَ قَدْ رَكَعَ فَكَبَّرْتَ قَبْلَ أَنْ يَرْفَعَ الْإِمَامُ رَأْسَهُ فَقَدْ أَدْرَكْتَ الرَّكْعَةَ وَ إِنْ رَفَعَ رَأْسَهُ قَبْلَ أَنْ تَرْكَعَ فَقَدْ فَاتَتْكَ الرَّكْعَةُ.
English translationBab Ul Qaim Publications
Hadith.1150 - Al-Halabi narrated from Imam Jafar ibn Muhammad Al-Sadiq (as) that He said: "If you reach the Imam while he is in the bowing position (ruku') and you say the takbir (opening supplication) before the Imam raises his head, then you have caught the rak'ah (unit of prayer). However, if he raises his head before you bow, then you have missed that rak'ah."
Translation: Bab Ul Qaim Publications·Thaqalayn link·The Arabic above is the original.