كتاب الزكاة
Obligatory Charity Tax (Zakat)
Chapter: The wealthy only are required to give in charity
باب لاَ صَدَقَةَ إِلاَّ عَنْ ظَهْرِ غِنًى
Sahih al-Bukhari 1427, 1428

Narrated Hakim bin Hizam:

The Prophet (ﷺ) said, "The upper hand is better than the lower hand (i.e. he who gives in charity is better than him who takes it). One should start giving first to his dependents. And the best object of charity is that which is given by a wealthy person (from the money which is left after his expenses). And whoever abstains from asking others for some financial help, Allah will give him and save him from asking others, Allah will make him self-sufficient."

حَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ، حَدَّثَنَا وُهَيْبٌ، حَدَّثَنَا هِشَامٌ، عَنْ أَبِيهِ، عَنْ حَكِيمِ بْنِ حِزَامٍ ـ رضى الله عنه ـ عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏ "‏ الْيَدُ الْعُلْيَا خَيْرٌ مِنَ الْيَدِ السُّفْلَى، وَابْدَأْ بِمَنْ تَعُولُ، وَخَيْرُ الصَّدَقَةِ عَنْ ظَهْرِ غِنًى، وَمَنْ يَسْتَعْفِفْ يُعِفَّهُ اللَّهُ، وَمَنْ يَسْتَغْنِ يُغْنِهِ اللَّهُ ‏"‏‏.‏ وَعَنْ وُهَيْبٍ، قَالَ أَخْبَرَنَا هِشَامٌ، عَنْ أَبِيهِ، عَنْ أَبِي هُرَيْرَةَ ـ رضى الله عنه ـ بِهَذَا‏.‏
Reference : Sahih al-Bukhari 1427, 1428
In-book reference : Book 24, Hadith 31
USC-MSA web (English) reference : Vol. 2, Book 24, Hadith 508
  (deprecated numbering scheme)