Sahih al-Bukhari, Book 24, Hadith 9 (In-Book) ⓘ

Narrated Khalid bin Aslam: We went out with 'Abdullah bin 'Umar and a bedouin said (to 'Abdullah), "Tell me about Allah's saying: "And those who hoard up gold and silver (Al-Kanz - money, gold, silver etc., the Zakat of which has not been paid) and spend it not in the Way of Allah (V.9:34)." 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."

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.‏