Al-Qasim narrates that a man asked his heirs for permission to bequeath more than one third, and they gave him permission; then, after he died, they went back on their permission. Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) was asked about this, and he said: "This going back (after granting permission) is a reprehensible act; it is not permissible."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9129