Nafi' (may Allah have mercy on him) narrates from Sayyiduna Ibn Umar (may Allah be pleased with them both) that he used to begin the recitation in the prayer with 'In the name of Allah, the Most Gracious, the Most Merciful' [Surah al-Fatihah: 1].
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2404