Makhul (may Allah have mercy on him) says that the Messenger of Allah (peace and blessings be upon him) said: "The good deed whose reward comes most quickly is maintaining ties of kinship, and the evil whose punishment comes most quickly is rebellion and transgression; and the false oath leaves homes desolate."
Imam al-Shafi'i (may Allah have mercy on him) said: Whoever deliberately swears falsely, saying: By Allah! such-and-such happened, when it did not, must give the expiation; he is a sinner and has done wrong by swearing falsely by Allah.
Imam al-Shafi'i (may Allah have mercy on him) said: If someone says: What is the proof that he must give the expiation when he intended a falsehood? He will be told: The closest proof is the saying of the Prophet (peace and blessings be upon him): "Let him do the better thing and give the expiation for his oath." So he commanded him to break his oath deliberately.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19872