Sayyidah Hafsah (may Allah be pleased with her), the wife of the Prophet (peace and blessings be upon him), narrates: I never saw the Messenger of Allah (peace and blessings be upon him) pray sitting, until one year 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 it became longer than a surah longer than it.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19643