How Did Fasting Become Obligatory?

 Here's a little history of Ramadan for general knowledge.

- Fasting the month of Ramadan was made obligatory (wajib) during the month of Sha'ban, in the second year after the Muslims migrated from Makkah to Madīnah. 

- Therefore, the Messenger of Allah (sallalaahu alayhi wasallam) by scholarly consensus (al-Ijma’), fasted the month of Ramadan nine times.

- The ordainment for fasting was in three stages:

The First Stage: The Muslims were ordered to fast on the day of Ashura (10th Muharram). [See Sahih Bukhari, Book #31]

The Second Stage: The Muslims were given the option of either fasting during the month of Ramadan or feeding a poor person for each day they did not fast, with preference for fasting over feeding. [Surah al-Baqarah (2): 184]

The Third Stage: Fasting the month of Ramaḍan became obligatory without choice. [Surah al-Baqarah (2): 185]. 

(Verse 2:184 was abrogated by verse 2:185 and also fasting on the day of Ashura became voluntary).

- Fasting during the month of Ramadan then became one of the pillars of Islam, and its obligation is of great magnitude. The Quran, Prophetic tradition, and scholarly agreement all testify to the fact that fasting during the month of Ramadan is obligatory.

- Why was it prescribed? The main wisdom behind the ordainment of fasting is to achieve taqwa. [Surah al-Baqarah (2): 183]

Reference: Fasting by Dr. Saleh As-Saleh (may Allah have mercy on him)