'Call them by [the names of] their fathers; it is more just in the sight of Allah,' meaning: more equitable. 'But if you do not know their fathers, then they are your brothers in religion and your mawali (those in your guardianship),' meaning: say, 'Our guardian is so-and-so,' and 'Our brother is so-and-so.' 'And there is no blame upon you for that in which you have erred,' meaning: sin. If a man errs after the prohibition and attributes a person to the one he had adopted, doing so out of forgetfulness, then there is no sin upon him for that. 'But [there is blame for] what your hearts have intended,' regarding calling them by names other than those of their fathers.

Surah al-Ahzab (Verse 6).