The fourth pillar of Islam is the Ramadan fast, during which Muslims abstain from eating, drinking, and sexual relations or satisfaction from dawn until sunset. Concerning the order to fast, the Qur'an declares:

The month of Ramadan, in which the Qur'an (began to be) sent down as a pure source of guidance for people, and, (when practiced,) as clear signs of guidance and the Criterion (between truth and falsehood). Therefore, whoever of you is present at this month must fast it, and he who is so ill that he cannot fast or is on a journey must fast the same number of other days. God desires ease for you, and desires not hardship for you, so that you can complete the number of the days required, exalt God for that He has guided you, and it is hoped that you may give thanks (due to Him). (2:185)