Sahih Bukhari, Chapter: Obligatory Charity Tax (Zakat)

When the verses of charity were revealed, we used to work as porters. A man came and distributed objects of charity in abundance. And they (the people) said, "He is showing off." And another man came and gave a sa (a small measure of food grains); they said, "Allah is not in need of this small amount of charity." And then the Divine Inspiration came: "Those who criticize such of the believers who give in charity voluntarily and those who could not find to give in charity except what is available to them." (9.79)