Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: The Messenger of Allah (peace and blessings be upon him) did not know the end of a surah until "Bismillahir-Rahmanir-Rahim (In the name of Allah, the Most Gracious, the Most Merciful)" was revealed to him (peace and blessings be upon him).
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12491