Sitting and Standing in the Night Prayer (Tahajjud)
he (sallallaahu 'alaihi wa sallam), used to pray long through the night standing, and long through the night sitting, and if he recited standing, he would bow standing, and if he recited sitting, he would bow sitting."23
sometimes, "he would pray sitting, so he would recite sitting until about thirty or forty verses of his recitation wre left; he would then stand up to recite these standing and then bow and prostrate, and he would do likewise in the second raka'ah."24
in fact, "he prayed as-subhah25 sitting down towards the end of his life when he had grown old, and that was a year before his death."26
also "he would sit cross-legged."27
23 muslim and abu daawood.
24 bukhaari and muslim.
25i.e. voluntary prayer (night or forenoon), named so due to its content of tasbeeh (glorification).
26 muslim and ahmad.
27 nasaa'i, ibn khuzaimah in his saheeh (1/107/2), `abdul ghani al-maqdisi in his sunan (80/1) & haakim, who declared it saheeh and dhahabi agreed.