Ahmad bin Shabeeb bin Saeed said that my father (Shabeeb bin Saeed) narrated to us, from Yunus, from Ibn Shihab, and from him Khalid bin Aslam, that we went out with Abdullah bin Umar (may Allah be pleased with them), and he said that this (aforementioned verse) was revealed before the command of Zakat. Then, when the command of Zakat was given, Allah the Exalted purified the wealth through Zakat.

Narrated Khalid bin Aslam: We went out with 'Abdullah bin 'Umar and 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 out 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."