It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that when the Prophet (peace and blessings be upon him) led the people in prayer, he would recite 'In the name of Allah, the Most Gracious, the Most Merciful' [Surah al-Fatihah: 1].
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2395