From Sahih Bukhari (Hadith 4668):
Narrated Abu Mas`ud:When we were ordered to give alms, we began to work as porters (to earn something we could give in charity). Abu `Aqil 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. Abu `Aqil); 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
Reference: Bukhari 4668