It is narrated from Umm al-Mu'mineen Sayyidah Hafsah (may Allah be pleased with her): I never saw the Messenger of Allah (peace and blessings be upon him) performing voluntary prayers while sitting, until one year before his passing, he began to perform voluntary prayers while sitting. And whenever he recited a surah, he would recite it with such measured recitation and pauses that it would become very lengthy.
Hadith Referenceالسنن المأثورة / ما جاء في الجمع بين الصلاتين في المطر / 26
Hadith Takhrijصحیح مسلم، المساجد ومواضع الصلاة، باب جواز النافلة قائمًا وقاعدًا، رقم: 733