Question: Does committing sins or backbiting break the fast?
No, it does not, but one should refrain from sins more strictly especially during fasting. A hadith-i sharif says, "Backbiting, spreading rumor, false swearing, looking lustfully at na-mahram break the fast" (Daylami).
Hadrat Imam A'zam explains this hadith-i sharif and states, "These sins vitiate the reward of fasting, but they do not vitiate its validity. They render the fast makruh." That is, even though those who commit these sins are absolved of the debt of fasting, they cannot attain the great reward peculiar to the practice of fasting. It is stated in another hadith-i sharif, "There is many a one who observes the fast but gains nothing but hunger and thirst" (Ibn Maja).
Fasting is a blessing and a trust for Believers. One must be faithful to a trust and observe its conditions in order for it not to be lost. One must abstain from looking at what is haram. A hadith-i sharif declares, "Looking at haram is one of Satan's poisonous arrows. Whoever leaves it out of fear of Allah, Allahu ta'ala grants him such a faith that he relishes its taste in his heart" (Hakim).
Besides, a fasting person must keep his tongue in check. A hadith-i sharif relates, "Fasting is a shield against the Fire. It protects as long as it is not ripped to pieces by backbiting. Let a fasting person not use obscene language in an act of ignorance. If anyone teases him, let him say, "I am fasting" (Bukhari).
As we protect our eyes and tongues from sins, so we must protect our ears from them because it is prohibited (haram) as well to listen to what is prohibited to talk about. It is necessary to protect our hands, feet, and other organs from prohibited actions, too. A person who is fasting but also is committing sins is like a patient who is taking poison instead of drug, for sins are poison. They wipe out our rewards that we earn in return for our acts of worship. After committing a sin, we should feel a pang of remorse and keep offering good deeds and acts of worship.