Yahya related to me from Malik from Ismail ibn Muhammad ibn Said ibn Abi Waqqas from a mawla of Amr ibn al-As or of Abdullah ibn Amr ibn al-As that the Messenger of Allah ﷺ said, "The prayer of one of you sitting down is only equal to half the prayer of one of you when he is standing."  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ صَلاَةُ أَحَدِكُمْ وَهُوَ قَاعِدٌ مِثْلُ نِصْفِ صَلاَتِهِ وَهُوَ قَائِمٌ  

Yahya related to me from Malik from Ibn Shihab that Abdullah ibn Amr ibn al As said, "When we arrived at Madina we were struck down by a severe epidemic which debilitated us greatly. The Messenger of Allah ﷺ came out to the people while they were praying nawafil prayers sitting down. The Messenger of Allah ﷺ said, 'The prayer of the one sitting is equal to only half the prayer of the one standing.' "  

أَنَّهُ قَالَ لَمَّا قَدِمْنَا الْمَدِينَةَ نَالَنَا وَبَاءٌ مِنْ وَعْكِهَا شَدِيدٌ فَخَرَجَ رَسُولُ اللَّهِ ﷺ عَلَى النَّاسِ وَهُمْ يُصَلُّونَ فِي سُبْحَتِهِمْ قُعُودًا فَقَالَ رَسُولُ اللَّهِ ﷺ صَلاَةُ الْقَاعِدِ مِثْلُ نِصْفِ صَلاَةِ الْقَائِمِ