Baillie says that the acknowledgement by a man of a child is valid under the fol­lowing circumstances: first, the ages of the parties must admit of the party acknowledged being born to the acknowledger: as otherwise he would be manifestly a liar. Second, the descent of the person acknowledged must not be already established from another; for if it were so, that would prevent its establishment from any bat him. 3rd, he must confirm the acknowledger in his acknowledgement if he can give an account of himself; for one he can do so in his own power contrary to the case of a child who cannot account for himself. (A Di­gest of Mohammadan Law—Baillie P 405) … In Durrul Mukhtar it is stated thus: "(i) If a person makes an acknowledge­ment in favour of a stranger whose pare­ntage is unknown and there after acknowledges him to be his son, and the latter verifies it which he is one of those who are fit to make such a verification, his descent is established with reference to the time of his being begotten. (ii) If a person make an acknowledge­ment in favour of a youth whose paren­tage is unknown whether at his birth place or In the town where he is, that the latter is his son and the acknowledgers and the acknowledged are such in age that the latter may be born of the former as a son, and the youth verifies it, he being capable of discretion (because otherwise his verification is not necessary as already stated) thus his descent is established, though the acknowledger be in sickness; and when the descent is so established, the youth will participate with the other heir". (CS.10 All 312). Hedaya sums up the law upon the subject- "If a person acknowledged parentage of a child who is able to give an account of himself, saying ''This is my son" and the ages of the parties be such as to admit of the one being the child of the other, and the parentage of the child be not well known to any person. It is well settled that the acknowledgement of parentage under the Mohammedan Law rests upon a footing higher than that of ordinary admission as pure matters of evidence. The rule of acknowledgement of paren­tage is based upon the words of Al-Quaran—Call them after their father". "(Sura 33.4) Muslim Jurists have tes­tified to this and the author of the Kifaya explains that such acknowledge­ment is sufficient because the burden of the obligation in respect of the child rests especially on the father, and the latter acknowledgement affects him per­sonally and is thus accepted without any verification by the mother." ... An author celebrated commentary on the Hedaya Fathual Qadir in explaining the rule quoted above as to the condition that the acknowledged child should be of unknown parentage, goes on to say that the condition has been imposed because otherwise the des­cent of the acknowledged child could not be established from the acknowledger since "des­cent after it is established is not susceptible of the annulment." In other words the acknowledged child should be of unknown paren­tage. ... 'Acknowledgement' in Mohammadan Law is no substitute for 'adop­tion’ in Hindu Law or Roman Law. Under Mohammad Law relationship by consanguinity is a pre-requisite for legitimacy and the latter it dependent upon a valid marriage of the parents. 

( Khorshed Alam @ Shah Alam Vs. Amir Sultan Ali Hyder and another, 1984, 13 CLC (AD) [1613] )