Bad speech during the month of Ramadhaan
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Question: Does impermissible (Haraam) speech during the day in Ramadhaan nullify the fast?

Response: If we read the saying of Allaah (subhaanahu wa-ta'aala), {O you who believe! Fasting is prescribed for you as it was prescribed for those before you, that you may become al-Muttaqoon (the pious)} [Soorah al-Baqarah, Aayah 183], we find the wisdom behind the obligation to fast is attaining fear of Allaah (Taqwah) and worshipping Allaah. Taqwah is to abandon all that is haraam and incorporating all that is commanded, avoiding all that is warned against. The Prophet (sal-Allaahu 'alayhe wa-sallaam) said: ((One who does not abandon evil talk and actions, then Allaah is not in need of him abandoning his food and drink (i.e. his fasting)). Based upon this, it is confirmation (for the one who is fasting) to abandon all that is haraam from speech and actions so as not to backbite, lie, slander or trade in haraam. So, keep away from haraam, and if a person were to do this throughout the entire month (of Ramadhaan), he would find himself upright for the rest of the year. Sadly, however, many people who fast do not differentiate between the time in which they fast and when they aren't It has became a norm whereby they indulge in haraam speech, lying, cheating and so on. They don't feel upon the peace and tranquility of the fast. These actions (of bad speech etc.) do not nullify the fast but do reduce the reward and, possibly, deflects the reward for the fast completely and Allaah's Assistance is sought.

Shaykh Ibn 'Uthaymeen
Faatwa Ramadhaan, Volume 2, Page 520, Fatwa No. 485;
Fataawa ash-Shaykh Muhammad as-Saalih al-'Uthaymeen - Volume 1, Page 501-502