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.
Hadith Referenceسنن دارمي / من كتاب الوصايا / 3286