Qatadah (may Allah have mercy on him) said: I asked Anas (may Allah be pleased with him) about the recitation of the Messenger of Allah (peace and blessings be upon him), and he said that he (peace and blessings be upon him) used to recite prolonging (the sounds).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2422