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.‏