Sahih al-Bukhari 4661
Sahih al-Bukhari · Book 65: Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh)) · The Statement of Allah "On the Day when that (Al-Kanz—money gold and silver, etc., the Zakät of which has not been paid) will be heated in the fire of Hell, and with it will be branded their foreheads…" (V.9:35)
Narrated Khãlid bin Aslam:
وَقَالَ أَحْمَدُ بْنُ شَبِيبِ بْنِ سَعِيدٍ حَدَّثَنَا أَبِي، عَنْ يُونُسَ، عَنِ ابْنِ شِهَابٍ، عَنْ خَالِدِ بْنِ أَسْلَمَ، قَالَ خَرَجْنَا مَعَ عَبْدِ اللَّهِ بْنِ عُمَرَ فَقَالَ هَذَا قَبْلَ أَنْ تُنْزَلَ، الزَّكَاةُ، فَلَمَّا أُنْزِلَتْ جَعَلَهَا اللَّهُ طُهْرًا لِلأَمْوَالِ.
We went out with 'Abdullãh bin 'Umar and he said, "This (Verse) was revealed before the prescription of Zakat, and when Zakãt was prescribed, Allah made it a means of purifying one's wealth."