When was the 2.5% zakat ordained, and what are the proofs for that from the Qur'an and the Sunnah?

The nisab (definition) (minimum taxable amount) for gold and silver zakat (definition) is one-quarter of one-tenth (2.5%), meaning five dirhams for every two hundred dirhams of silver, and half a dinar for every twenty dinars of gold. Any amount exceeding this is calculated proportionally. This also applies to trade goods and currently circulating currency. Scholars have reached a consensus on this specific amount.

Zakat was mandated in Mecca before the Hijra (emigration to Medina), but the determination of its nisabs, amounts, and detailed rulings was established in Medina in the second year of the Hijra.