It is narrated from the Mother of the Believers Sayyidah Umm Salamah (may Allah be pleased with her) that the recitation of the Prophet (peace and blessings be upon him) was "'In the name of Allah, the Most Gracious, the Most Merciful' [Surah al-Fatihah: 1]", and Affan, describing the manner of his (peace and blessings be upon him) recitation, said that he (peace and blessings be upon him) would recite making each letter distinct and would prolong the sound of every letter.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2423