Sayyidah Hafsah (may Allah be pleased with her), the wife of the Prophet (peace and blessings be upon him), says: I did not see the Messenger of Allah (peace and blessings be upon him) sitting in voluntary prayer, 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; he (peace and blessings be upon him) would recite a surah so slowly and deliberately that its recitation became longer than his (peace and blessings be upon him) recitation while standing.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19646