(10) باب اتَّقُوا النَّارَ وَلَوْ بِشِقِّ تَمْرَةٍ
Narrated Abu Mas`ud:
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).
Reference: Sahih al-Bukhari 1415
In-book reference: Book 24, Hadith 19
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 2, Book 24, Hadith 496