Sahih Muslim
The Book of Virtue, Good Manners and Joining of the Ties of Relationship (Kitab Al-Birr was-Salat-I-wal-Adab)
Book Number: 032 / Hadith Number: 6309
'Abdullah reported Allah's Messenger (may peace be upon him) as saying: It is obligatory for you to tell the truth, for truth leads to virtue and virtue leads to Paradise, and the man who continues to speak the truth and endeavours to tell the truth is eventually recorded as truthful with Allah, and beware of telling of a lie for telling of a lie leads to obscenity and obscenity leads to Hell-Fire, and the person who keeps telling lies and endeavours to tell a lie is recorded as a liar with Allah.