Being two-faced — praising a person to his face while disparaging him behind his back, or presenting a different self to each group in order to win favour with both — is condemned in the strongest terms. The Prophet (peace be upon him) said: "You will find the worst of people in the sight of Allah on the Day of Resurrection to be the two-faced one, who comes to these with one face and to those with another face" (Sahih al-Bukhari 6058). Scholars class this as a form of practical hypocrisy (nifaq 'amali), of the same family as the signs the Prophet listed: "The signs of a hypocrite are three: when he speaks he lies, when he promises he breaks it, and when he is entrusted he betrays" (Sahih al-Bukhari 33). Practical hypocrisy is a grave sin that a believer must root out of himself, but it is not the hypocrisy of creed that puts a person outside Islam. This also does not mean a Muslim should be blunt or harsh: being gentle, warm and pleasant with everyone is praised, and speaking softly to someone in order to soften his heart is not two-facedness. What is condemned is deceit — saying what one does not mean, and playing people off against one another.
Q&A · Character
What does Islam say about being two-faced (dhul-wajhayn)?
Evidence — the daleel
Sahih al-Bukhari 6058 ṢaḥīḥSahih al-Bukhari 33 Ṣaḥīḥ
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Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.