Sahih al-Bukhari — Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh)) — Hadith No: 4661
Translation
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