Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: Jibril (peace be upon him) used to bring down the revelation to the Messenger of Allah (peace and blessings be upon him), and when he said "Bismillahir-Rahmanir-Rahim (In the name of Allah, the Most Gracious, the Most Merciful)", a new surah began (with it).
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12493