It is narrated from Nafi' that when Abdullah bin Umar (may Allah be pleased with them both) began the recitation in prayer, he would recite 'Bismillahir-Rahmanir-Rahim' (In the name of Allah, the Most Gracious, the Most Merciful). Then, when he finished Surah al-Fatihah, he would also recite 'Bismillah' (In the name of Allah) when beginning the next surah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3057