The Quran

لَيسَ عَلَى الضُّعَفاءِ وَلا عَلَى المَرضىٰ وَلا عَلَى الَّذينَ لا يَجِدونَ ما يُنفِقونَ حَرَجٌ إِذا نَصَحوا لِلَّهِ وَرَسولِهِ ۚ ما عَلَى المُحسِنينَ مِن سَبيلٍ ۚ وَاللَّهُ غَفورٌ رَحيمٌ

Muhammad Asad

[But] no blame shall attach to the weak,¹ nor to the sick, nor to those who have no means. [to equip themselves],² provided that they are sincere towards God and His Apostle: there is no cause to reproach the doers of good, for God is much-forgiving, a dispenser of grace.

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.