The Prophet ﷺ would not know where one surah ended and the next began until "In the name of Allah, the Most Gracious, the Most Merciful" was revealed to him. This is the wording of Ibn Sarh.
حدثنا قتيبة بن سعيد وأحمد بن محمد المروزي وابن السرح قالوا حدثنا سفيان عن عمرو عن سعيد بن جبير قال قتيبة فيه
كان النبي ﷺ لا يعرف فصل السورة حتى تنزل عليه بسم الله الرحمن الرحيم وهذا لفظ ابن السرح
[Sahih] Bayhaqi narrated it in 2/42, 43 from the hadith of Abu Dawud through him. Humaydi also narrated it, no. 528, as did Nasai in al-Kubra, no. 11636, and Tahawi in Mushkil Athar, 2/153. Hakim narrated it in 1/231, and Dhahabi said, "As for this, it is established."