Hadrat Abu Dharr al-Ghaffari (r.a.) reported: ‘I heard the Messenger of Allah (saws) saying: "A person who attributes his fatherhood to anyone other than his real father, knowing that he is not his father, commits an act of disbelief. And he who makes a claim of anything which in fact does not belong to him, is none of us. He should make his abode in Hell, and he who labels anyone as disbeliever or calls him the enemy of Allah and he is in fact not so, his charge will revert to him.''’
Related by Bukhari and Muslim. quoted from Riyad-us-Saliheen (Hadith # 1805)
Amongst the grave sins which would be tantamount to an act of ‘kufr’ or disbelief in the Sight of Allah Subhanah are:
1. To intentionally attribute one’s lineage or fatherhood to a person who is not one’s biological father.
2. To falsely claim something which one knows does not rightfully belong to them.
3. To label or accuse a true believer of disbelief, or accuse them of being an enemy of Allah.