The Prophet ﷺ arrived in Madinah while it was stricken with fever, and the people fell ill with the fever. The Prophet ﷺ entered the mosque and found the people sitting while praying. The Prophet ﷺ said, "The prayer of the one who sits is half the prayer of the one who stands." So the people forced themselves to pray standing.
حدثنا محمد بن بكر قال حدثنا ابن جريج قال قال ابن شهاب
قدم النبي ﷺ المدينة وهي محمة فحم الناس فدخل النبي ﷺ المسجد والناس قعود يصلون فقال النبي ﷺ صلاة القاعد نصف صلاة القائم فتجشم الناس الصلاة قياما
Its isnad is sahih. It is found in Muslim, 1/507, no. 735, in the chapter on travelers, on the permissibility of the voluntary prayer while standing or sitting, and in Nasai, 3/223, no. 1660, in the chapter on night prayer, on the merit of praying standing, and in Ibn Majah, 1/388, no. 1229, from Ibn Amr, and in Malik, 1/137, in the chapter on congregational prayer, also from Ibn Amr.