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.
Reported by Abu Dawud (No. 788). Cited in Mishkat al-Masabih (No. 2218).
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.