It is permissible to offer voluntary prayers standing or sitting, and to stand and sit in the same rak`ah

'Abdullah b. Shaqiq said: I asked 'A'isha about the Messenger of Allah's (ﷺ) voluntary prayers, and she replied: Before the noon prayer, he used to pray four rak'ahs in my house; then would go out and lead the people in prayer; then come in and pray two rak'ahs. He would then lead the people in the sunset prayer; then come in and pray two rak'ahs. Then he would lead the people in the 'Isha' prayer, and enter my house and pray two rak'ahs. He would pray nine rak'ahs during the night, including Witr. At night he would pray for a long time standing and for a long time sitting, and when he recited the Holy Qur'an while standing, he would bow and prostrate himself from the standing position, and when he recited while sitting, he would bow and prostrate himself from the sitting position, and when it was dawn he would pray two rak'ahs.