Fasting is one of the pillars of Islam and whoever does not fast has abandoned one of the pillars and is committing a grave major sin. Indeed some of the salaf were of the view that he is a kaafir [disbeliever] and apostate (we seek refuge with Allaah from that) because Allaah says (interpretation of the meaning):
“O you who believe! Observing As-Sawm (the fasting) is prescribed for you as it was prescribed for those before you, that you may become Al-Muttaqoon (the pious).” [al-Baqarah 2:183]
The prophet (peace and blessings of Allaah be upon him) also said: “Islam is built on five (pillars): the testimony that there is no god but Allaah and Muhammad is the Messenger of Allaah; establishing prayer; paying zakaah; Hajj; and fasting Ramadaan.”
In another narration, it he (peace and blessings of Allaah be upon him) said, “Whilst I was sleeping two men came to me and took my by the arm and brought me to a cragged mountain. They said, ‘Climb up.’ I said, ‘I cannot.’ They said, ‘We will make it easy for you.’ So I climbed up until I was at the top of the mountain. Then I heard loud voices. I said, ‘What are these voices?’ They said, ‘This is the howling of the people of Hell.” Then I was taken until I saw people hanging by their hamstrings, with the sides of their mouths torn and blood pouring from their mouths.’ I said, ‘Who are these?’ He said, ‘These are people who broke their fast before it was time.’”
Al-Dhahabi said, “Whoever does not fast in Ramadaan without being sick or having any other excuse that allows him not to fast is more evil than the adulterer or drunkard, indeed they doubted his Islam and thought that this was heresy.
…Abide by the truth even though your mind deserts you.