Al-Muttalib bin Abi Wada'ah al-Sahmi narrates from Sayyidah Hafsah (may Allah be pleased with her) that she said: I never saw the Prophet (peace and blessings be upon him) pray a voluntary prayer sitting until a year before his death, when he would pray voluntary prayer sitting. And he would recite a surah in slow, measured recitation (tartil), until it became longer than a surah longer than it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4587