Sayyidah Hafsah (may Allah be pleased with her) says: I never saw the Messenger of Allah (peace and blessings be upon him) pray sitting, until one or two years before his (peace and blessings be upon him) death, when he (peace and blessings be upon him) began to pray voluntary prayer sitting; and he would recite a surah so slowly and deliberately that his (peace and blessings be upon him) recitation became longer than that of a surah longer than it.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19642