It is narrated from Sayyiduna Anas (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) used to recite "Bismillahir-Rahmanir-Rahim" quietly, and Sayyiduna Abu Bakr and Sayyiduna Umar (may Allah be pleased with them both) did the same.
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 735