كتاب صلاة المسافرين وقصرها
6  
The Book of Prayer - Travellers
(16)
Chapter: It is permissible to offer voluntary prayers standing or sitting, and to stand and sit in the same rak`ah
(16)
باب جَوَازِ النَّافِلَةِ قَائِمًا وَقَاعِدًا وَفِعْلِ بَعْضِ الرَّكْعَةِ قَائِمًا وَبَعْضِهَا قَاعِدًا ‏
Sahih Muslim 735 a

'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.
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 ‏"‏ ‏.‏
Reference : Sahih Muslim 735a
In-book reference : Book 6, Hadith 145
USC-MSA web (English) reference : Book 4, Hadith 1600
  (deprecated numbering scheme)