From Sahih Bukhari (Hadith 5046):
Narrated Qatada:Anas was asked, "How was the recitation (of the Qur'an) of the Prophet?' He replied, "It was characterized by the prolongation of certain sounds." He then recited: In the Name of Allah, the Most Beneficent, the Most Merciful prolonging the pronunciation of 'In the Name of Allah, 'the most Beneficent,' and 'the Most Merciful
Reference: Bukhari 5046