The biggest sin of tongue according to the Prophet (PBUH)
The Prophet of Islam (PBUH) said in his sermon: One of the biggest sins of the tongue is to lie . A person came to the Prophet (PBUH) and said: I do not pray, I do adultery and I do lie, which one should I abandon first? The Prophet (PBUH) replied: Lying. That person promised to the Prophet (PBUH) to abandon lying but when he left, the Satan tempted him into adultery. He suddenly thought if tomorrow the Prophet asked him about this and if he said he had not done it, then he would be lying and if he would tell the truth then he will be eligible to be punished according to Sharia Law. Therefore this way of thinking and abandoning lying is the start of leaving behind all sins .
 Beharul Anwaar, Vol. 21, Page 211.
 Tafseer Nemoneh, Vol. 11, Page 413.