Hafsah Radiyallahu 'Anha, the wife of Rasulullah Sallallahu 'Alayhi Wasallam reports:
" Rasulullah Sallallahu 'Alayhi Wasallam performed while sitting. He recited surah in it, and recited with such tarteel (distinct and clear intonation), that the surah became longer than one that is lengthier".
English reference: Book 39, Hadith 265
Arabic reference: Book 40, Hadith 281