I read a narration mentionned in some books of tarikh. Some khawarij said to ali you are a kafir and he is said to have replied yeah i am kafir.The way he is said to have replied it was clear that he was saying it in a sarcastic way and not as form of joking or accepting the speech but as a way of rejecting it. I wanted to know what is the hukm of doing this because a lot of people say words of kufr this way.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

It is well-known that the Khawaarij declared ‘Ali to be a disbeliever, because of him appointing Abu Moosa Al-Ash’ari and ‘Amr Ibn Al-‘Aas to judge between him and Mu’aawiyah all.

As regards him saying to them what you mentioned in the question, then this is a matter that we have not come across [in the references that we have at hand].

The scholars stated that if one says the word of Kufr [disbelief] deliberately while knowing that it is a word of Kufr, he becomes a Kaafir [disbeliever]. But whoever says: “He is a Kaafir” (referring to himself) in a sarcastic way while denouncing it, then he does not become a Kaafir because he is actually denying being so.

In any case, a Muslim should avoid delusive terms especially those related to Kufr as someone may hear him while not knowing his objective in saying it, and so he may think evil of him and thus he might be harmed because of this.

Allaah Knows best.