When was the Ramadan fast mandated?

The fasting of Ramadan was mandated in the month of Sha'ban in the second year of the Hijra. The evidence for its obligation is Allah's saying: "O you who have believed, prescribed for you is fasting as it was prescribed for those before you, that you may become righteous" [Quran 2:183], and the Prophet, peace and blessings be upon him, said: "Islam is built upon five [pillars]: the testimony that there is no god worthy of worship except Allah and that Muhammad is the Messenger of Allah, establishing prayer, giving zakat, fasting Ramadan, and performing Hajj to the House for whoever is able to find a way to it."