Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1597
Hafsa reported: Never did I see the Messenger of Allah (may peace be upon him) observing supererogatory prayer sitting till one year before his death when he would observe Nafl prayer in a sitting position, and he would recite the Surah (of the Qur'an) in such a slow-measured tone (that duration of its recital) became more lengthy than the one longer than this.