Surah 9. Al-Tawba, Ayah 91

I.e., the old and the infirm.

Lit., "who do not find anything to spend", i.e., on their equipment. At the time in question a public treasury did not yet exist, and every participant in a military expedition was expected to provide his own weapons and mounts.

Though active service in person or by contributing resources is expected in emergencies of every person who believes in the Cause, there are some who must necessarily be exempted without the least blame attaching to them. Such would be those who are weak in body on account of age, sex, infirmity, or illness. Personal service in their case is out of the question, but they could contribute towards expenses if they are able. But if they are too poor to afford even such assistance, they are excused. But in all cases the motive must be sincere, and there should be a desire to serve and do such duty as they can. With such motives people are doing good or right in whatever form they express their service: sometimes, in Milton's words, "they also serve who only stand and wait." In any case their purity of motive would get Allah's grace and forgiveness, and we must not criticise even if we thought they might have done more.