It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Muslims would not know of the end of a surah until "In the name of Allah, the Most Gracious, the Ever-Merciful" was revealed, and when "In the name of Allah, the Most Gracious, the Most Merciful" was revealed, they understood that the surah was complete.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2378
Hadith Gradingمحدثین:صحيح
Hadith Takhrij«صحيح۔ اخرجه اخرجه الطحاوي في مشكل الآثار 3/406»