It is permissible to offer voluntary prayers standing or sitting, and to stand and sit in the same rak`ah
وَحَدَّثَنِي زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا جَرِيرٌ، عَنْ مَنْصُورٍ، عَنْ هِلاَلِ بْنِ يَسَافٍ، عَنْ أَبِي يَحْيَى، عَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو، قَالَ حُدِّثْتُ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " صَلاَةُ الرَّجُلِ قَاعِدًا نِصْفُ الصَّلاَةِ " . قَالَ فَأَتَيْتُهُ فَوَجَدْتُهُ يُصَلِّي جَالِسًا فَوَضَعْتُ يَدِي عَلَى رَأْسِهِ فَقَالَ مَا لَكَ يَا عَبْدَ اللَّهِ بْنَ عَمْرٍو قُلْتُ حُدِّثْتُ يَا رَسُولَ اللَّهِ أَنَّكَ قُلْتَ " صَلاَةُ الرَّجُلِ قَاعِدًا عَلَى نِصْفِ الصَّلاَةِ " . وَأَنْتَ تُصَلِّي قَاعِدًا قَالَ " أَجَلْ وَلَكِنِّي لَسْتُ كَأَحَدٍ مِنْكُمْ " .
'Abdullah b. 'Amr reported: It was narrated to me that the Messenger of Allah (ﷺ) had said: The prayer observed by a person sitting is half of the prayer. I came to him (ﷺ) and found him praying in a sitting position. I placed my hand on his head. He said: O 'Abdullah b. 'Amr, what is the matter with you? I said: Messenger of Allah, it has been narrated to me that you said: The prayer of a man in a sitting position is half of the prayer, whereas you are observing prayer sitting. He (the Holy Prophet) said: Yes, it is so, but I am not like anyone amongst you.