Of the primary hallmarks of a believer (Episode 17 in our series) is the purity of his speech: the believer avoids lying, slander, backbiting, gossip, vulgarity, and argumentation. Multiple times in the Quran, Allah describes the believers as those who '...turn away from idle talk,' or those who '...when the foolish address them, respond by saying: Peace!'.
The Prophet (SAW) said, 'Whoever guarantees me [the control] of what is between his cheeks, and what is between his thighs, I shall in return guarantee him Paradise" [Tirmidhi]. This means the two primary causes of failure are what the tongue reaps, and the following of one's sensual pleasures. In another tradition, we are told that the harvest of the tongue is a primary reason for punishment.
So many problems of this world are caused by speech: just look at how many relationships have been broken, how much corruption has spread, how much deceit has taken place, and even how wars have been started and lives lost, and you will see that evil speech is a primary factor in all of that.
Hence why the believer prefers silence over vain and useless speech.The Prophet (SAW) reminded us: "Whoever remains silent is safe" [Tirmidhi].