The Prophet (sas) said:
مَنْ كَانَ يُؤْمِنُ بِاللَّهِ وَالْيَوْمِ الْآخِرِ فَلَا يُؤْذِ جَارَهُ وَمَنْ كَانَ يُؤْمِنُ بِاللَّهِ وَالْيَوْمِ الْآخِرِ فَلْيُكْرِمْ ضَيْفَهُ وَمَنْ كَانَ يُؤْمِنُ بِاللَّهِ وَالْيَوْمِ الْآخِرِ فَلْيَقُلْ خَيْرًا أَوْ لِيَصْمُتْ
Whoever believes in Allah and in the last day, let him never annoy his neighbor. And whoever believes in Allah and in the last day, let him generously honor his guest. And whoever believes in Allah and in the last day, let him speak that which is good or else keep quiet.
... أَلَا أُخْبِرُكَ بِمَلَاكِ ذَلِكَ كُلِّهِ قُلْتُ بَلَى يَا نَبِيَّ اللَّهِ فَأَخَذَ بِلِسَانِهِ قَالَ كُفَّ عَلَيْكَ هَذَا فَقُلْتُ يَا نَبِيَّ اللَّهِ وَإِنَّا لَمُؤَاخَذُونَ بِمَا نَتَكَلَّمُ بِهِ فَقَالَ ثَكِلَتْكَ أُمُّكَ يَا مُعَاذُ وَهَلْ يَكُبُّ النَّاسَ فِي النَّارِ عَلَى وُجُوهِهِمْ أَوْ عَلَى مَنَاخِرِهِمْ إِلَّا حَصَائِدُ أَلْسِنَتِهِمْ
[After describing all of the basics of Islam such as the five pillars, jihad, etc., the Prophet (sas) continued:] ... Should I not inform you of that which eenables and strengthens all of that? I said, "Why yes, O Prophet of Allah!" He (sas) then took hold of his tongueand said, Restrain this. I said, "Are we then held accountable for what we [merely] talk about? He (sas) said, May your mother be bereaved O Muadh! And, are the people thrown on their faces into the fire for any reason other than the fruits of their tongues?
...يَا عَائِشَةُ إِنَّ مِنْ شِرَارِ النَّاسِ الَّذِينَ يُكْرَمُونَ اتِّقَاءَ أَلْسِنَتِهِمْ
...O Aisha, the worst people are those who are honored only for protection from their tongues.
Allah requires of us that we REFRAIN from speaking when that is the best thing to do or when our speaking will be haram or its harm will outweigh its benefits. But, He also requires us to SPEAK when that is what is called for and when our speaking is obligatory or when the benefits of speaking will outweigh any harm.
Some Muslims go to extremes in this and come to view silence as absolute good and extoll the virtues of silence over speaking. This then becomes an excuse for isolation and cutting one's self off from the Muslims. This is a falsehood, a form of extremism (if well-intentioned) and an innovation in relgion. If you look to books which advocate this type of position, you will find them quoting many "hadith" and other texts, but these hadith invariably turn out to be weak and false. Nonetheless, their intention was to remind the Muslims that there are many times when the temptation is to speak with that which is not lawful and at such times, one should maintain silence and restrain the tongue as the above hadith make clear. As one author put it:
"Afflictions [of the tongue] are many and varied. They have a sweetness in the heart. And supporting them are drives in human nature. [Sometimes] there is not safety from them except keeping quiet."
If you realize the preciousness of your allotted time and that it is you only capital [to invest for your future in the hereafter], then you would NEVER spend any of it in that which is of no benefit. This knowledge will also without doubt hold your tongue back from that which does not concern you. The Prophet (sas) said:
من حسن اسلام المرء تركه ما لا يعنيه
Part of the completion of your Islam is to leave that which does not concern you.
There are many types of falsehood or sin which one could enter into in speaking. The Prophet (sas) said:
إِنَّ الْعَبْدَ لَيَتَكَلَّمُ بِالْكَلِمَةِ مِنْ رِضْوَانِ اللَّهِ لَا يُلْقِي لَهَا بَالًا يَرْفَعُهُ اللَّهُ بِهَا دَرَجَاتٍ وَإِنَّ الْعَبْدَ لَيَتَكَلَّمُ بِالْكَلِمَةِ مِنْ سَخَطِ اللَّهِ لَا يُلْقِي لَهَا بَالًا يَهْوِي بِهَا فِي جَهَنَّمَ
A slave will speak a word of that which pleases Allah without attaching any importance to it but with which Allah raises him many levels. And a slave will speak a word of that which Allah hates without attaching any importance to it but by which he tumbles down into hell-fire.
Also from this category is arguing. The intention here is arguing not to discover the truth or Allah's ruling in a matter but rather arguing in order to "defeat" another and to show that his position is weak and one's own position is strong. The Prophet (sas) said:
إِنَّ أَبْغَضَ الرِّجَالِ إِلَى اللَّهِ الْأَلَدُّ الْخَصِمُ
The most hateful of men to Allah is the ever-argumentative one [who goes to excess in argumentation].