Narrated Abu Musud:

When we were ordered to give alms, we began to work as porters (to earn something we could give in charity). Abu Uqail came with one half of a Sa (special measure for food grains) and another person brought more than he did. So the hypocrites said, "Allah is not in need of the alms of this (i.e. 'Uqail); and this other person did not give alms but for showing off." Then Allah revealed:-- 'Those who criticize such of the Believers who give charity voluntarily and those who could not find to give in charity except what is available to them.' (9.79)