Allaah’s Messenger صلى الله عليه وسلم said, “No one who is disobedient to his parents, no one who reminds others of his favours, no drunkard, and no fornicator [lit: 'son of fornication'] will enter Paradise.” Reported by Nisaa’i and others.
The Imaam said, “His saying, ‘… and no son of fornication will enter Paradise …’ is not to be taken upon its apparent meaning, rather what is meant is the person who fornicates such that it becomes his overwhelming characteristic, and so as a result of that he deserves to be attributed to it, and thus it is said of him, ‘He is the son of fornication,’ just as those who are addicted/love the dunyaa are attributed to it, it being said of them, ‘Sons of the world,’ due to their knowledge and their addiction/love of it, and just as the traveller is called, ‘Son of the road,’ [cf: Surah Baqarah 2:177, 'ابن السبيل'].
So, ‘the son of fornication,’ is like these expressions, it is said of the one who is given to fornication such that its habituality has become attributed to him, and fornication has become his overwhelming characteristic, such a person is the one referred to in his saying, ‘… and no son of fornication will enter Paradise …’
The child who is born out of fornication and who himself is not a fornicator is not what is meant here … and I benefitted by acquiring this meaning from the speech of Ibn Ja’far at-Tahaawi, may Allaah have mercy on him, and his explanation of this hadith.
And Allaah knows best.