Such sins do not invalidate the fast in the technical sense — the day is still counted and you do not repeat it — but they strip it of its reward and defeat its purpose. The Prophet ﷺ said: "Whoever does not give up false speech and acting upon it, Allah has no need of his giving up his food and drink." He also instructed: "Fasting is a shield, so when one of you is fasting let him not speak obscenely or behave foolishly, and if someone insults him or fights him, let him say: I am fasting." Scholars draw the obvious conclusion: the fast of the stomach is the minimum, and a person can end the day hungry with little or nothing to show for it. So the fasting person guards the tongue, the eye and the limbs as seriously as he guards his stomach — and if he slips, he repents and continues the fast rather than abandoning it.
Q&A · Fasting
Does backbiting, lying, or losing my temper break my fast?
Evidence — the daleel
Sahih al-Bukhari 1903 ṢaḥīḥSahih Muslim 1151 Ṣaḥīḥ
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.