It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) would not recognise the end of a surah until "In the name of Allah, the Most Gracious, the Most Merciful" was revealed to him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2377