Sahih al-Bukhari 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.' (9.79)
حَدَّثَنِي بِشْرُ بْنُ خَالِدٍ أَبُو مُحَمَّدٍ، أَخْبَرَنَا مُحَمَّدُ بْنُ جَعْفَرٍ، عَنْ شُعْبَةَ، عَنْ سُلَيْمَانَ، عَنْ أَبِي وَائِلٍ، عَنْ أَبِي مَسْعُودٍ، قَالَ لَمَّا أُمِرْنَا بِالصَّدَقَةِ كُنَّا نَتَحَامَلُ فَجَاءَ أَبُو عَقِيلٍ بِنِصْفِ صَاعٍ، وَجَاءَ إِنْسَانٌ بِأَكْثَرَ مِنْهُ، فَقَالَ الْمُنَافِقُونَ إِنَّ اللَّهَ لَغَنِيٌّ عَنْ صَدَقَةِ هَذَا، وَمَا فَعَلَ هَذَا الآخَرُ إِلاَّ رِئَاءً‏.‏ فَنَزَلَتْ ‏{‏الَّذِينَ يَلْمِزُونَ الْمُطَّوِّعِينَ مِنَ الْمُؤْمِنِينَ فِي الصَّدَقَاتِ وَالَّذِينَ لاَ يَجِدُونَ إِلاَّ جُهْدَهُمْ‏}‏ الآيَةَ‏.‏