كتاب التفسير
65
Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh))
(7)
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)
(7)
Sahih al-Bukhari 4661

Narrated Khãlid bin Aslam:

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."

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.‏
Reference : Sahih al-Bukhari 4661
In-book reference : Book 65, Hadith 183
USC-MSA web (English) reference : Vol. 1, Book 60, Hadith 183
  (deprecated numbering scheme)