Q. What is the legal ruling for fasting in the month of Ramaḍān?
A. Fasting in the month of Ramaḍān is mandatory as attested by the texts: the Qur’an, the Sunnah, and the consensus of Muslims.
Allāh states: ‘You who believe, fasting is prescribed for you, as it was prescribed before you, so that you may be mindful of Allāh’ up until ‘It was the month of Ramaḍān that the Qur’an was revealed as guidance for mankind, clear messages giving guidance and distinguishing between right and wrong. So any one of you who sees in that month should fast…’ (Q 2: 183-6).
The Prophet states: ‘Islām is built on five [components]: testifying that there is none worthy of worship but Allāh and that Muḥammad is Allāh’s Messenger, establishing the prayer, giving alms, fasting Ramaḍān, and making pilgrimage to Allāh’s Sacred House’ (Bukhārī, 8 & Muslim, 16 – with different wordings).
The Prophet also states: ‘When you see it [the moon] then fast’ (Bukhārī 1900, Muslim 1080, 1081).
The Muslims are in unanimity that fasting in Ramaḍān is mandatory and that it is one of the pillars of Islam.
Thus, whoever denies its obligation disbelieves, unless they are brought up in a far away land and are not aware of the legal rulings of Islām – then they must be informed or educated. If, however, they persist [in not acknowledging its obligation] after the proofs have been established, then they have disbelieved. Moreover, whoever abandons this fasting while neglecting its obligation; they place themselves in a dangerous position. Here, some of the scholars would view such a person as an apostate – but the most preferred position is that they are not an apostate; instead they are a wicked person (fāsiq) due to their dangerous position.
Shaykh Ibn ʿUthaymīn – Majmūʿ al-Fatāwā wa Rasāʾil (11/19)