Making false allegation against an innocent person is on of the meanest misdeeds that have been severely censured by Islam.
Verses
1. The Holy Quran says: And whoever commits a fault or a sin, then accuses of it one innocent, he indeed takes upon himself the burden of a calumny and a manifest sin.
2. And those who speak evil things of the believing men and the believing women without their having earned it, they are guilty indeed of a false accusation and a manifest sin.
Traditions
Imam Sadiq (a.s.) said: Making allegation against an innocent is higher than even the high hills in sinning.
Imam Sadiq (a.s.) also said: The one who falsely damages a Muslim brother makes the faith in his heart melt away like the melting of salt in water.
And the Holy Prophet (s.a.w.s.) has said: If one puts a false allegation on a believing man or a believing woman or says anything about them which is not is him or her, the Lord almighty will, on the Day of Judgement, keep him in the cloud of hell-smoke until he comes out of the responsibility of what he had said.
Imam Sadiq (a.s.) said: If one makes an allegation against a believing man or woman saying what is not in him or her, God Almighty, keeps him in the Teenat-e-Khabaal until what he had said comes out. The narrator says: I asked: What is Teenat Khabaal? He replied: It is a dirty pus which comes out of the private part of a bad character woman.
Briefly speaking it should be remembered that slander and defamation or false allegation is one of the worst kind of lies because it contains the great evil of falsity as well as the harms of back-biting. Moreover it is the worst kind of oppression.
It must also be kept in mind that the spread of this cowardly misdeed in a society becomes the cause of the dislocation of social justice and indulgence in falsity and entanglement of the innocent and the freeing of the criminal and the loss of collective trust.