Assalamu alaikum wr wb
It is reported in al-Musnad, on the authority of Anas, that the Prophet said: "The faith of a servant is not put right until his heart is put right, and his heart is not put right until his tongue is put right." This shows that the Prophet has made the purification of faith conditional on the purification of the heart, and the purification of the heart conditional on the purification of the tongue.
At-Tirmidhi relates in a hadith on the authority of Ibn 'Umar: "Do not talk excessively without remembering Allah, because such excessive talk without the mention of Allah causes the heart to harden, and the person furthest from Allah is a person with a hard heart."
'Umar Ibn al-Khattab, may Allah be pleased with him, said: "A person who talks too much is a person who often makes mistakes, and someone who often makes mistakes, often has wrong actions. The Fire has a priority over such a frequent sinner."
In a hadith related on the authority of Mu'adh, the Prophet said, "Shall I not tell you how to control all that?" I said, "Yes do, O Messenger of Allah." So he held his tongue between his fingers, and then he said: "Restrain this." I said, "O Prophet of Allah, are we accountable for what we say?" He said, "May your mother be bereft by your loss! Is there anything more than the harvest of the tongues that throws people on their faces (or he said "on their noses") into the Fire?"
What is meant here by "he harvest of the tongues'" is the punishment for saying forbidden things. A man, through his actions and words, sows the seeds of either good or evil. On the Day of Resurrection he harvests their fruits. Those who sow the seeds of good words and deeds harvest honour and blessings; those who sow the seeds of evil words and deeds reap only regret and remorse.
A hadith related by Abu Huraira says, "What mostly causes people to be sent to the Fire are the two openings: the mouth and the private parts."
Abu Huraira also related that the Messenger of Allah said, "The servant speaks words, the consequences of which he does not realise, and for which he is sent down into the depths of the Fire further than the distance between the east and the west."
The same hadith was transmitted by at-Tirmidhi with slight variations: "The servant says something that he thinks is harmless, and for which he will be plunged into the depths of the Fire as far as seventy autumns."
Uqba ibn Amir said: "I said: "O Messenger of Allah, what is our best way of surviving?" He, may Allah bless him and grant him peace, replied: "Guard your tongue, make your house suffice for sheltering your privacy, and weep for your wrong actions.""
It has been related on the authority of Sahl ibn Sa'd that the Prophet said, "Whoever can guarantee what is between his jaws and what is between his legs, I guarantee him the Garden."
It has also been related by Abu Huraira, may Allah be pleased with him, that the Prophet, may Allah bless him and grant him peace, said, "Let whoever believes in Allah and the Last Day either speak good or remain silent."
Thus talking can either be good, in which case it is commendable, or bad, in which case it is haram.
The Prophet said: "Everything the children of Adam say goes against them, except for their enjoining good and forbidding evil, and remembering Allah, Glorious and Might is He." This was reported by at-Tirmidhi and Ibn Majah on the authority of Umm Habiba, may Allah be pleased with her.
Umar ibn al-Khattab visited Abu Bakr, may Allah be pleased with them, and found him pulling his tongue with his fingers. Umar said "Stop! may Allah forgive you!" Abu Bakr replied; "This tongue has brought me to dangerous places."
Abdullah ibn Masud said: "By Allah, besides Whom no god exists, nothing deserves a long prison sentence more than my tongue." He also used to say: "O tongue, say good and you will profit; desist from saying evil things and you will be safe; otherwise you will find only regret."
Abu Huraira reported that Ibn al-Abbas said: "A person will not feel greater fury or anger for any part of his body on the Day of Judgement more than what he will feel for his tongue, unless he only used it for saying or enjoining good."
Al-Hassan said: "Whoever does not hold his tongue cannot understand his deen." The least harmful of a tongue's faults is talking about whatever does not concern it. The following hadith of the Prophet is enough to indicate the harm of this fault: "One of the merits of a person's Islam is his abandoning what does not concern him."
Abu Ubaida related that al-Hassan said: "One of the signs of Allah's abandoning a servant is His making him preoccupied with what does not concern him." Sahl said, "Whoever talks about what does not concern him is deprived of truthfulness."
As we have already mentioned above, this is the least harmful of the tongue's faults. There are far worse things, like backbiting, gossiping, obscene and misleading talk, two-faced and hypocritical talk, showing off, quarrelling, bickering, singing, lying, mockery, derision and falsehood; and there are many more faults which can affect a servant's tongue, ruining his heart and causing him to lose both his happiness and pleasure in this life, and his success and profit in the next life. Allah is the One to Whom we turn for assistance.