Sahih Bukhari Volume 2, Book 24, Hadith 1405

وَقَالَ أَحْمَدُ بْنُ شَبِيبِ بْنِ سَعِيدٍ حَدَّثَنَا أَبِي، عَنْ يُونُسَ، عَنِ ابْنِ شِهَابٍ، عَنْ خَالِدِ بْنِ أَسْلَمَ، قَالَ خَرَجْنَا مَعَ عَبْدِ اللَّهِ بْنِ عُمَرَ ـ رضى الله عنهما ـ فَقَالَ أَعْرَابِيٌّ أَخْبِرْنِي قَوْلَ اللَّهِ، ‏{‏وَالَّذِينَ يَكْنِزُونَ الذَّهَبَ وَالْفِضَّةَ وَلاَ يُنْفِقُونَهَا فِي سَبِيلِ اللَّهِ‏}‏ قَالَ ابْنُ عُمَرَ ـ رضى الله عنهما ـ مَنْ كَنَزَهَا فَلَمْ يُؤَدِّ زَكَاتَهَا فَوَيْلٌ لَهُ، إِنَّمَا كَانَ هَذَا قَبْلَ أَنْ تُنْزَلَ الزَّكَاةُ فَلَمَّا أُنْزِلَتْ جَعَلَهَا اللَّهُ طُهْرًا لِلأَمْوَالِ‏

And Narrated Khalid bin Aslam: We went out with Abdullahbin Umar (R.A.) and a bedouin said(to Abdullah),"Tell me about Allah's saying:'They who hoard up gold and silver and spend them not in the way of Allah." Ibn Umar said,"Whoever hoarded them and did not pay the Zakat thereof ,then woe to him. But these holy verses were revealed before the verses of Zakat.So when the verses of Zakat were revealed Allah made Zakat a purifier of the property."