Sayyida Hafsa (may Allah be pleased with her) narrates: I never saw the Prophet Muhammad (peace be upon him) performing voluntary prayers while sitting, until one year before his passing, the Prophet (peace be upon him) began to perform voluntary prayers while sitting. If he recited any Surah, he would recite it slowly and distinctly, so much so that it would seem longer than a Surah greater than it.
Hadith Referenceصحیح ابن حبان / كتاب الصلاة / 2508
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح - «صفة الصلاة»، «مختصر الشمائل» (237): م.فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده صحيح على شرط مسلم.