"كان لا يعرف فصل السورة حتى ينزل عليه {بِسْمِ اللَّهِ الرَّحْمَنِ الرَّحِيمِ}
It was the practice of the Prophet (peace and blessings be upon him):
"He (peace and blessings be upon him) would not know that a surah had ended until 'Bismillahir-Rahmanir-Rahim' (In the name of Allah, the Most Gracious, the Most Merciful) was revealed to him."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4864
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[د] عن ابن عباس. صحيح أبي داود ٧٥٤: الطحاوي، ك، هق، الضياء.