Mishkat al-Masabih 2218
Arabic hadith text
وَعَنِ ابْنِ عَبَّاسٍ قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ لَا يَعْرِفُ فَصْلَ السُّورَةِ حَتَّى يَنْزِلَ عَلَيْهِ بِسْمِ اللَّهِ الرَّحْمَنِ الرَّحِيم. رَوَاهُ أَبُو دَاوُد
Translation
Ibn ‘Abbās said that God's messenger did not know where sūras were to be separated till “In the name of God, the Compassionate, the Merciful,” was sent down. Abū Dāwūd transmitted it.