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

Ibn ‘Abbās (may Allah be pleased with him) points out that the Qur’an's surahs would reveal to the Prophet (may Allah's peace and blessings be upon him) while he did not know where they should end and be separated, until "In the name of Allah, the Most Compassionate, the Most Merciful" was revealed to him; thereupon, he knew that the previous surah came to an end and this was the beginning of a new surah.