Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.003.

It is acceptable for one to make nawafil while sitting even though he has the ability to stand. It is also acceptable for one to make part of such prayers sitting and part of them standing even if all of that is in one rak’ah, (i.e., one sits for part of the first rak’ah and then stands for the rest of it, or vice versa). All of that is acceptable without any dislike for it. One may sit in any manner one likes although it is preferable to sit cross-legged. Muslim records that ‘Alqamah asked ‘Aishah: “How did the Prophet perform two rak’at while sitting?” She replied: “He would recite while sitting and then when he wished to make ruku’, he would stand and bow.” Ahmad, Abu Dawud, at-Tirmizhi, an-Nasa’i, and Ibn Majah record that she said: “I never saw the Messenger of Allah ever sitting while reciting during the night prayer until he became old, then he would sit until when about thirty or forty verses were left of his recital then he would stand, finish the recital and make ruku’…”