It was narrated that 'Abdullah bin 'Amr said: "I saw the Prophet (ﷺ) praying sitting down and I said: 'I was told that you said that the prayer of one who is sitting down is worth half of the prayer of the one who is standing up.' He said: 'Yes indeed, but I am not like any one of you.'"
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا عُبَيْدُ اللَّهِ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا يَحْيَى، عَنْ سُفْيَانَ، قَالَ حَدَّثَنَا مَنْصُورٌ، عَنْ هِلاَلِ بْنِ يِسَافٍ، عَنْ أَبِي يَحْيَى، عَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو، قَالَ رَأَيْتُ النَّبِيَّ صلى الله عليه وسلم يُصَلِّي جَالِسًا فَقُلْتُ حُدِّثْتُ أَنَّكَ قُلْتَ " إِنَّ صَلاَةَ الْقَاعِدِ عَلَى النِّصْفِ مِنْ صَلاَةِ الْقَائِمِ " . وَأَنْتَ تُصَلِّي قَاعِدًا . قَالَ " أَجَلْ وَلَكِنِّي لَسْتُ كَأَحَدٍ مِنْكُمْ " .
sunnah.com