They were a group during the time of the Messenger of Allah ﷺ who did not go out on military campaigns with him due to illnesses, diseases, and ailments. Others were healthy yet still did not join him on campaigns. The sick had a valid excuse, unlike the healthy.
في قول الله عزوجل: {لا يستوي القعدون من المؤمنين غير أولى الضرر}
هم قوم كانوا على عهد رسول الله ﷺ لايغزون معه لأسقام، وأمراض، وأوجاع، وآخرون أصحاء لا يغزون معه، وكان المرضي في عذر من الأصحاء.
Tabarani narrated it in Al-Kabir (12/165), and Bayhaqi (9/24), both through routes from Abu Aqeel Dawraqi (that is, Bashir bin Uqba Naji), from Abu Nadra (that is, Mundhir bin Malik Abdi), from Ibn Abbas, and he related it. Its chain of transmission is sahih.Haythami said, Tabarani narrated it through two routes, and the men of one of them are trustworthy. Majma al-Zawaid (7/9).