Are there any valuable shlokas in the Vedas, especially in Dharma sastras, which warn liars or ones who deceive people? And I'd like to know what will happen to a liar and deceitful person after death. Also what will happen to one who lie to a brahmana or a saintly person like a guru.
In Hinduism, sins are divided into three categories -- 1. mental sins (sins committed by thoughts) 2. verbal sins (committed by speech) and 3. bodily sins (those committed by actions).
In this classification, lying comes under type 2 i.e. sins committed by verbal actions.
Manu Smiti has the relevant verses:
12.5. Coveting the property of others, thinking in one’s heart of what is undesirable, and adherence to false (doctrines), are the three kinds of (sinful) mental action.
12.6. Abusing (others, speaking) untruth, detracting from the merits of all men, and talking idly, shall be the four kinds of (evil) verbal action.
And, similarly there are sins that fall under type3. Now, all these sins have certain consequences.
For example, verbal sins result in that Jiva getting born as a beast or a bird.
12.9. In consequence of (many) sinful acts committed with his body, a man becomes (in the next birth) something inanimate, in consequence (of sins) committed by speech, a bird, or a beast, and in consequence of mental (sins he is re-born in) a low caste.
Also what will be happened to one who lie to brahmana or saintly person like guru.
Lying to Guru (or lying in front of Guru) is a much bigger sin than what lying in any other situation is.
Lord Shiva says the following:
sAmAnyato nishidhancha tad-guro yadi sannidhau |
Acharettasya sarvashya doshah koti-guno bhavet ||
(An action) What is generally prohibited, if done in the vicinity of the Guru, assumes the form of a sin of a much bigger proportion.
KulArnava Tantram 12.97
Therefore. if lying is a sin, lying in front of Guru is a much bigger sin. The verse uses "koti-guno bhavet" i.e. "multiplied by a crore" which means the sin gets magnified manifold times.