The Messenger of Allah - SallAllahu 'alaihi wa sallam - said, “Guard your tongue, stay in your homes and weep over your sins.” - Related by Ibn Mubarak in az-Zuhd (no.134), Musnad Ahmed bin Hambal (5/259), Sunan at-Tirmidhi (2531) and authenticated by al-Albaanee in as-Saheehah (no.890)

Also the Messenger of Allah - SallAllahu 'alaihi wa sallam - said, “Most of the sins of the children of Aadam are from the tongue.” - At-Tabaraanee (3/87/1-2). Authenticated by al-Haafidh al-Mundhiri in al-Targheeb (4/8)

Narrated by Mughira bin Shu’ba - radiAllahu' anhu - : The Prophet - SallAllahu 'alaihi wa sallam - said, 
“Allah has forbidden for you:(1) to be undutiful to your mothers, (2) to bury your daughters alive, (3) not to pay the rights of the poor and others (i.e. charity) and (4) to beg of men. And Allah has hated for you: (1) Qil and Qal (sinful and useless talk, like backbiting or that one talks too much about others), (2) to ask too many questions (in disputed religious matters, etc.), (3) to waste your wealth (by extravagance with lack of wisdom and thinking).” - Sahih al-Bukhari [3:591]

Abu Moosa al-Asharee - radiAllahu' anhu - said, “O Messenger of Allah, which of the Muslims are better?” The Messenger of Allah said, “Those who do not harm the Muslims with their tongues and hands.” - Saheeh al-Bukhari (1/10) and Saheeh al-Muslim (1/64)

Sahl ibn Sa’ad - radiAllahu' anhu - reported that the Messenger of Allah said, “Whoever guards what is between his jaws and legs. I shall guarantee him paradise.” - Saheeh al-Bukhari (8/481). A similar narration can be found in Muwatta of Imam Malik (56/11) and Sunan at-Tirmidhi.

When the Companion of the Prophet, Mu’adh ibn Jabal - radiAllahu' anhu - asked the Prophet - SallAllahu 'alaihi wa sallam - about that which would entitle him to enter Paradise and save him from the hell-fire, he - SallAllahu 'alaihi wa sallam - mentioned the pillars of Islam and informed Mu’adh of the importance of the night prayers and of Jihaad. He - SallAllahu 'alaihi wa sallam - then said, “Should I not inform you of the sheet anchor of all this?”, and then took hold of his tongue and said, “Exercise restraint on it.”. Mu’adh - radiAllahu' anhu - enquired, “O Messenger of Allah, will we be held responsible for what we say with it?” Thereupon, the Messenger of Allah said, “Mu’adh, will anything else besides (irresponsible) talk cause the people to be thrown in the Hell-Fire upon their faces?” - Sunan at-Tirmidhi and Riyaadh as-Saaliheen (2/1522)

Most of the people who will enter the fire, will do so due to their tongues 
So a Muslim must beware and learn to imprison the tongue from vain talk, for the Messenger of Allah - SallAllahu 'alaihi wa sallam - said, “Every saying of the children of Aadam is cursed, expept for the enjoining good and forbidding the evil or for the remembrance of Allah.” - Sunan at-Tirmidhi

And “The world and all that it contains is cursed, except for the remembrance of Allah and what supports it,and a scholar and a student.” - Sunan Ibn Majah (no.4112) and and authenticated by Sheikh al-Albani in Saheehul-Jaami’ (no.3414)

Abu Hurairah - radiAllahu' anhu - reports on the authority of the Prophet - SallAllahu 'alaihi wa sallam - that, “From the good Islaam of a man, is that he leaves that which does not benefit him.” - Musnad Ahmed bin Hambal (4/132), Sunan at-Tirmidhi, Sunan Ibn Majah, al-Baihaqi in Shu’ab al-Imaan