Sayyidah Hafsah (may Allah be pleased with her) says: I never saw the Messenger of Allah (peace and blessings be upon him) pray voluntary prayer sitting, until one or two years before his (peace and blessings be upon him) death, when I saw him (peace and blessings be upon him) praying voluntary prayer sitting; and he (peace and blessings be upon him) would recite a surah so slowly and deliberately that its recitation became longer than that of a surah longer than it.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19644