[Machine] The Muslims did not know the end of the chapter until "Bismillah ir-Rahman ir-Rahim" was revealed. When "Bismillah ir-Rahman ir-Rahim" was revealed, they knew that the chapter had ended, and so on.
كَانَ الْمُسْلِمُونَ لَا يَعْلَمُونَ انْقِضَاءَ السُّورَةِ حَتَّى تَنْزِلَ بِسْمِ اللهِ الرَّحْمَنِ الرَّحِيمِ فَإِذَا نَزَلَتْ بِسْمِ اللهِ الرَّحْمَنِ الرَّحِيمِ عَلِمُوا أَنَّ السُّورَةَ قَدِ انْقَضَتْ وَكَذَلِكَ