Sunan an-Nasa'i, Book 11, Hadith 139 (In-Book) ⓘ
It was narrated that Qatadah said: "I asked Anas: 'How did the Messenger of Allah recite Quran?' He said: 'He used to elongate the sounds.'"
أَخْبَرَنَا عَمْرُو بْنُ عَلِيٍّ، قَالَ حَدَّثَنَا عَبْدُ الرَّحْمَنِ، قَالَ حَدَّثَنَا جَرِيرُ بْنُ حَازِمٍ، عَنْ قَتَادَةَ، قَالَ سَأَلْتُ أَنَسًا كَيْفَ كَانَتْ قِرَاءَةُ رَسُولِ اللَّهِ صلى الله عليه وسلم قَالَ كَانَ يَمُدُّ صَوْتَهَ مَدًّا .
also referenced under: Hadith 1014 (Reference) & Volume 2, Book 11, Hadith 1015 (DarusSalam)