Ibrahim (may Allah have mercy on him) said: When a man makes a bequest for someone that he should be given a share like that of his son, then his share will not be complete until there is a reduction from the son's share.

It is narrated from Imam Sha'bi, may Allah have mercy on him: If a man has three sons and he makes a bequest for someone equal to the share of one of them, as if he had four sons, Sha'bi, may Allah have mercy on him, said: He will be given one-fifth.

Dawud bin Abi Hind said: We asked ‘Amir (Sha’bi, may Allah have mercy on him): A man left behind two sons and made a will that so-and-so should be given a share equal to one of their shares, when they are three? Sha’bi, may Allah have mercy on him, said: I instruct that he be given a quarter share. In some versions it is «أوصي بالربع» and in others «افتي بالربع» ‏‏‏‏, meaning he gave the fatwa of a quarter.

Ibrahim (may Allah have mercy on him) said: If a man makes a bequest (wasiyyah) for someone equal to the share of one of his heirs, he said: Such (ambiguous) bequest, even if it is less than one third, is not permissible. Imam Darimi (may Allah have mercy on him) said: This statement is good.