Allah says in the Quran:
بِئْسَ الِاسْمُ الْفُسُوقُ بَعْدَ الْإِيمَانِ
(Wretched is the name of fisq after having faith. [Quran 49:11])
Allah says that the name of fisq is wretched and disgusting to say after someone has had faith. What is fisq? Fisq is anything that is an act of disobedience, anything that is shameless and unethical. Allah is saying that any form of “rubbish” is wretched to say, especially after having faith.
A Massive Challenge
In today’s society, controlling the tongue is a massive challenge. The use of the f-word as an adjective for almost everything, good or bad, is commonplace. When confronted people usually reply, “I am expressing myself!”
Is there no other way to express oneself? Especially in times of anger, our mouths go into a tirade of foul-mouthed attacks on the people around us. Little do we consider that this might be a problem for ourselves in this world, in our relationships with people, and most important a problem in our relationship with Allah.
The Prophet sallallahu alayhi wasallam says in an amazing hadith that the rest of the body addressed the tongue, saying, Fear Allah concerning us, for we are (dependant) upon you. If you are upright then we will be upright and if you are corrupt then we will be corrupt.  It is interesting that the Prophet equated uprightness of the whole body to the uprightness of the tongue! And as Allah mentioned it is “wretched” to mention anything filthy after having faith.
Allah says about the successful believers in the Quran:
Indeed successful are the Believers, those who in their prayer have Khushoo’ (concentration and mindfulness of Allah) and those who refrain from vain talk. 
Allah mentions that the first quality of successful believers is that they are mindful of Allah in their prayers, and the second quality is that they refrain from irrelevant talk. Allah has combined belief, success, prayer and guarding of the tongue in almost the same sentence! Allah further says:
Not a word is said except that there is a watcher by him ready to record it. 
Every word we say is recorded. Why? Because words are also part of our scroll of deeds, and for every word we can be rewarded or punished. For every word we will be questioned as to why we said it! And that is why the Prophet sallallahu alayhi wasallam said, Whoever believes in Allah and the Last Day should speak a good word or remain silent. 
Allah also tells us that a good word might as much as help us earn forgiveness and mercy from Him! He says:
O you who believe! Be conscious of Allah and speak a word that is good. He will set right for you your deeds and forgive you your sins. And whoever obeys Allah and His Messenger has certainly attained a great achievement. 
And the Prophet sallallahu alayhi wasallam also gave a great warning in what he terms “the harvests of the tongue”, when he said, Does anything topple people headlong into the Hellfire save the harvests of their tongues? 
The Prophet also said, sallallahu alayhi wasallam, Indeed a servant will speak a word pleasing to Allah that he thinks to be insignificant, but because of it Allah raises him by many degrees. And indeed a servant will speak a word displeasing to Allah that he thinks to be insignificant, but because of it, He will consign him to the Hellfire. 
Therefore it is imperative that we control our tongues. How do you go about achieving that? Here are four easy things you can do:
- Think before you speak; if you don’t have something nice to say, don’t say anything at all.
- Apologize if you have said something that is bad or wrong.
- Surround yourself with good company— people who speak well and avoid evil speech.
- Increase your dhikr of Allah. Constant remembrance of Allah will remind us to keep all of our actions in check, and this includes our tongues.
Let us make a pledge to try and improve bit by bit the speech that comes out of our tongues For the reward for it is paradise. The Prophet sallallahu alayhi wasallam said:
Whoever guards what is between his jaws and legs. I shall guarantee him paradise. 
 Narrated by at-Tirmidhi
 Quran 23:1-3
 Quran 50:18
 Narrated by al-Bukhari and Muslim
 Quran 33:70-71
 Narrated by at-Tirmidhi and Ibn Majah
 Narrated by al-Bukhari
 Narrated by al-Bukhari