Surat Al-'Aĥzāb - سورة الأحزاب

ادْعُوهُمْ لِآبَائِهِمْ هُوَ أَقْسَطُ عِندَ اللَّهِ ۚ فَإِن لَّمْ تَعْلَمُوا آبَاءَهُمْ فَإِخْوَانُكُمْ فِي الدِّينِ وَمَوَالِيكُمْ ۚ وَلَيْسَ عَلَيْكُمْ جُنَاحٌ فِيمَا أَخْطَأْتُم بِهِ وَلَٰكِن مَّا تَعَمَّدَتْ قُلُوبُكُمْ ۚ وَكَانَ اللَّهُ غَفُورًا رَّحِيمًا

Muhsin Khan: Call them (adopted sons) by (the names of) their fathers, that is more just with Allah. But if you know not their father's (names, call them) your brothers in faith and Mawalikum (your freed slaves). And there is no sin on you if you make a mistake therein, except in regard to what your hearts deliberately intend. And Allah is Ever OftForgiving, Most Merciful.