Muwatta Imam Malik Book of Prayer in Congregation, Hadith 21

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.' "

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، عَنْ عَبْدِ اللَّهِ بْنِ عَمْرِو بْنِ الْعَاصِ، 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 " .