(19) باب صَلاَةِ الْقَاعِدِ فِي النَّافِلَةِ وَذِكْرِ الاِخْتِلاَفِ عَلَى أَبِي إِسْحَاقَ فِي ذَلِكَ
It was narrated that Hafsah said:
"I never saw the Messenger of Allah (ﷺ) offer his voluntary prayers sitting down until one year before his death. Then he used to pray sitting down, reciting the surah so slowly that it seemed to be longer than a surah that is longer."
Reference: Sunan an-Nasa'i 1658
In-book reference: Book 20, Hadith 61
English translation: Vol. 2, Book 20, Hadith 1659