Nafi' narrates from Sayyiduna Ibn Umar (may Allah be pleased with them both) that when Sayyiduna Ibn Umar (may Allah be pleased with them both) recited, he would recite "In the name of Allah, the Most Gracious, the Most Merciful" [Surah al-Fatihah: 1] aloud, and when he recited the next surah he would again recite the Bismillah aloud.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3058