Lying is The Biggest Sin

Abdullah ibn Mas’ud reported: The Messenger of Allah, peace and blessings be upon him, said, “Most of the sins of the children of Adam are on their tongues.”

Please Check The Ref, Numbers

"Whoever believes in Allah and the Last Day should say [something] good, or he should keep silent." [Bukhari, Muslim, Ahmad, Tirmidhi, Ibn Majah]

Rasoolullah SalLalLaho Alaihe Wasallam say kisi nay pocha:

Momin Buzdil ho sakta hai? Aap SalLalLaho Alaihe Wasallam nay Farmaya: Haan, Phir Pocha Kia Momin Bakheel ho Sakta hai? Aap Alaihe Salam nay Farmaya: Haan, Phir Pocha: Kia Momin Jhoota Ho Sakta hai? Aap Alaihe Salam nay Farmaya: Nahi

(Muotta Imaam Malik, Hadees # 2371, Kitab uL Jamay, Baab: Sach aur Jhoot ka Bayan, Ravi: Safwan Bin Sulaym Rz.A)

Hazrat Abu Hurairah (Radhiyallaahu Anhu) narrates, in a hadith, that the Prophet (Sallallaahu Alayhi Wasallam) said, "The signs of a hypocrite are three; whenever he speaks, he tells a lie; whenever he promises, he breaks his promise; if you have trust in him, he proves to be dishonest."

Hazrat Abu Bakr radiallaho ta'ala anhu narrates that Rasool Allah sallalaho alaihe wasallam said, "Should I tell you the greatest among the deadly sins," I said, "Why not? Ya Rasool Allah sallalaho alaihe wasallam." He said, " Associating partners with Allah Ta'ala, disobedience of Parents, (when he was saying this he was lying in a relaxed position, suddenly he sat straight up), and said, "Beware of lying"... he kept on repeating this for such a long time that we started hoping that he would end this topic and talk to us about another topic." (Bukhari Shareef, Vol 1)

Sunan Abu Dawud Book 41, Number 4953: Narrated Sufyan ibn Asid al-Hadrami: I heard the Apostle of Allah (peace_be_upon_him) say: It is great treachery that you should tell your brother something and have him believe you when you are lying .

Sunan Abu Dawud Book 41, Number 4972: Narrated Mu'awiyah ibn Jaydah al-Qushayri: The Apostle of Allah (peace_be_upon_him) said: Woe to him who tells things, speaking falsely, to make people laugh thereby. Woe to him! Woe to him !.

Prophet Muhammad [peace and blessings of Allah be upon him] said, “Whoever seeks the pleasure of Allah at the risk of displeasing the people, Allah will take care of him and protect him from them. But whoever seeks the pleasure at the risk of displeasing Allah, Allah will abandon him to the care of people.” [Reported by Tirmidhi. 4/34,... at the end of the section on zuhd; it is a Hasan Hadith.]

1515. Abu Hurairah (May Allah be pleased with him) reported: The Prophet (PBUH) said, "A man utters a word pleasing to Allah without considering it of any significance for which Allah exalts his ranks (in Jannah); another one speaks a word displeasing to Allah without considering it of any importance, and for this reason he will sink down into Hell.'' [Al-Bukhari].