Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
bayhaqi:2591Abū Bakr b. al-Ḥārith > Abū Muḥammad b. Ḥayyān > Ibrāhīm > Abū ʿĀmir > al-Walīd > Ismāʿīl > ʿAmr b. Muhājir > ʿUmar b. ʿAbd al-ʿAzīz

[Machine] If he catches them in a bowing position, he will say two Takbirs, one Takbir for the opening of the prayer, and one Takbir for the bowing, and he has caught the Rak'ah.  

البيهقي:٢٥٩١وَأَخْبَرَنَا أَبُو بَكْرِ بْنُ الْحَارِثِ أنبأ أَبُو مُحَمَّدِ بْنُ حَيَّانَ ثنا إِبْرَاهِيمُ ثنا أَبُو عَامِرٍ ثنا الْوَلِيدُ قَالَ وَأَخْبَرَنِي إِسْمَاعِيلُ عَنْ عَمْرِو بْنِ مُهَاجِرٍ عَنْ عُمَرَ بْنِ عَبْدِ الْعَزِيزِ قَالَ

إِذَا أَدْرَكَهُمْ رُكُوعًا كَبَّرَ تَكْبِيرَتَيْنِ تَكْبِيرَةً لِافْتِتَاحِ الصَّلَاةِ وَتَكْبِيرَةً لِلرُّكُوعِ وَقَدْ أَدْرَكَ الرَّكْعَةَ