وَ رُوِيَ عَنْ عَبْدِ اَلرَّحْمَنِ بْنِ أَبِي عَبْدِ اَللَّهِ أَنَّهُ قَالَ سَمِعْتُ أَبَا عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ يَقُولُ: «إِذَا دَخَلْتَ اَلْمَسْجِدَ وَ اَلْإِمَامُ رَاكِعٌ وَ ظَنَنْتَ أَنَّكَ إِنْ مَشَيْتَ إِلَيْهِ رَفَعَ رَأْسَهُ فَكَبِّرْ وَ اِرْكَعْ فَإِذَا رَفَعَ رَأْسَهُ فَاسْجُدْ مَكَانَكَ فَإِذَا قَامَ فَالْحَقْ بِالصَّفِّ وَ إِنْ جَلَسَ فَاجْلِسْ مَكَانَكَ فَإِذَا قَامَ فَالْحَقْ بِالصَّفِّ.
It was narrated from Abdul Rahman ibn Abi Abdullah that He said: "I heard Imam Jafar ibn Muhammad Al-Sadiq (a.s) say: "If you enter the mosque and the Imam is in the bowing position (ruku'), and you think that if you walk towards him, he will raise his head, then pronounce the takbir (opening supplication) and bow. When the Imam raises his head, prostrate in your place. If the Imam then stands, join the row. If he sits, remain seated in your place, and when he stands again, join the row."