Musannaf Ibn Abi Shaybah

Qatada narrates from Hazrat Saeed bin Musayyib regarding the man who says that a third of my wealth is for such-and-such person, and if he dies during my lifetime, then it is for another person; he (Saeed bin Musayyib) said: that wealth will be given to the first person.

Qatadah also narrates this from Hasan.

Qatadah narrates from Hazrat Humaid bin Abdur Rahman that the action will be carried out exactly as the one making the will has stated.

Hisham bin Urwah narrates the same subject from his noble father.