Abu Hurayrah, may Allaah be pleased with him, reported: “I heard the Prophet sallallaahu ‘alayhi wa sallam, saying: “A person utters a word thoughtlessly (i.e., without thinking about whether it is good or evil) and as a result, will plummet into the fire of Hell deeper than the distance between the east and the west.” [Al-Bukhaari & Muslim]
Abu Hurayrah, may Allaah be pleased with him, reported that the Prophet sallallaahu ‘alayhi wa sallam said: “A man may utter a word pleasing to Allaah without considering it of any significance, but for which Allaah elevates his ranks (in Paradise); another one may speak a word displeasing to Allaah without considering it of any importance, but for which he would be plummeted into Hell.” [Al-Bukhaari]
Abu ‘Abdur-Rahmaan Bilaal bin Al-Haarith Al-Muzani, may Allaah be pleased with him, reported that the Messenger of Allaah sallallaahu ‘alayhi wa sallam said: “A man may speak a good word without knowing its worth, for which Allaah records for him His good pleasure until the Day he will meet Him; another man may utter an evil word without realising its importance, for which Allaah records for him His displeasure until the Day he will meet Him.” [Maalik & At-Tirmithi]
When the Prophet sallallaahu ‘alayhi wa sallam was talking to Mu’aath, may Allaah be pleased with him, he told him: “The root (i.e. foundation) of this matter is Islaam, its pillar (mainstay) is the prayer and its apex is Jihaad (fighting in the cause of Allaah).” Then he sallallaahu ‘alayhi wa sallam asked: “Shall I tell you of that which holds together all these things?” Mu’aath, may Allaah be pleased with him, said: “Yes, O Messenger of Allaah!” So he sallallaahu ‘alayhi wa sallam held his tongue between his fingers, and said: “Restrain this.” Mu’aath, may Allaah be pleased with him, asked: “O Messenger of Allaah! Will we really be held to account for what we say?” The Prophet sallallaahu ‘alayhi wa sallam exclaimed: “May your mother lose you! Most people will be thrown on their faces into the Hellfire on account of their tongues.” [Ahmad & At-Tirmithi]
Ibn Mas’ood, may Allaah be pleased with him: “That which requires imprisonment the most is one’s tongue.”
`Adiyy bin Haatim, may Allaah be pleased with him, reported the Messenger of Allaah sallallaahu ‘alayhi wa sallam as saying: “Allaah will surely speak to every one of you without an interpreter. He (i.e., the one being spoken to) will look to his right and see nothing but (deeds) which he had done, and will look to his left and see nothing but (deeds) which he had done. Then he will look in front of him and will find nothing except the Hellfire facing him. So protect (yourselves) from the Fire, even if by giving in charity half a date, and if you cannot find that, then with a kind word.” [Al-Bukhaari & Muslim]
Silence is Golden…quotes from Nawawi’s Kitab Al-Adhkaar
Imam Shafi’ 
said: “When one wishes to say something, then it is upon him to think before he speaks. If there is beneficial good in what he will say, then he should speak. If he has doubt about that, then he must not speak until that doubt is removed.”
al-Fudayl ibn ‘Iyaad said: “Whoever limits his speech to be in accordance with his actions will (surely) lessen his speech on that which doesn’t concern him.”
Imam Shafi’ adviced one of his students: “Do not speak about things that do not concern you, for indeed, every time that you speak a word, it takes control of you and you do not have any control over it!”
Abdullah ibn Mas’ood 
said: “There is nothing that deserves to be imprisoned more than the tongue.”
Abul-Qaasim al-Qushayree said: “Remaining silent is salvation and that is a fundamental principle. Staying quiet at the appropriate time is a characteristic of manhood, just as speaking when it is right to do so is from the most honorable of manners.”
Islamic Etiquette 