Authentic
Sunan Abu Dawud 788
The Prophet (may Allah’s peace and blessings be upon him) did not know where Surahs were to be separated until "In the name of Allah, the Most Compassionate, the Most Merciful" was revealed to him.
لاَ يَعْرِفُ فَصْلَ السُّورَةِ حَتَّى تُنَزَّلَ عَلَيْهِ { بِسْمِ اللَّهِ الرَّحْمَنِ الرَّحِيمِ } .
Narrated by Ibn ‘Abbās + 1 other version Explained
