It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) used to recite "Bismillahir-Rahmanir-Rahim" (In the name of Allah, the Most Gracious, the Most Merciful) aloud.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11394