Translation:

5. Call them by (the names) of their fathers: that is juster in the sight of Allah but if ye know not their father's (names call them) your Brothers in faith or your Maulas. But there is no blame on you if ye make a mistake therein: (what counts is) the intention of your hearts: and Allah is Oft-Returning Most Merciful.

Notes (Tafseer)

3672. Freedmen were often called after their master's name as the "son of so and so". When they were slaves, perhaps their father's names were lost altogether. It is more correct to speak of them as the Maula of so and so. But Maula in Arabic might also imply a close relationship of friendship: in that case, too, it is better to use the right term instead of the term "son". "Brother" is not objectionable because "Brotherhood" is used in a wider sense than "fatherhood" and is not likely to be misunderstood.

3673. What is aimed at is to destroy the superstition of erecting false relationships to the detriment or loss of true blood relations. It is not intended to penalise an unintentional slip in the matter, and indeed, even if a man deliberately calls another his son or father, who is not his son or father, out of politeness or affection, "Allah is Oft- Forgiving, Most Merciful". It is the action of mischievous parties which is chiefly reprehended, if they intend false insinuations. A mere mistake on their part does not matter.