وَعَنِ ابْنِ عَبَّاسٍ
قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: لَا يَعْرِفُ فَصْلَ السُّورَةِ حَتَّى يَنْزِلَ عَلَيْهِ بِسْمِ اللَّهِ الرَّحْمَنِ الرَّحِيم
God’s messenger did not know where suras were to be separated till “In the name of God, the Compassionate, the Merciful,” was sent down. Abu Dawud transmitted it.
Mishkat al-Masabih 2122, Book 9, The Excellent Qualities of the Qur'an, Hadith 108. 1 min read