Cui bono? (/
The phrase is a double dative construction. It can also be rendered as cui prodest? ("whom does it profit?") and ad cuius bonum? ("for whose good?").
L. Cassius ille, quem populus Romanus verissimum et sapientissimum iudicem putabat, identidem in causis quaerere solebat, cui bono fuisset?
Lucius Cassius, whom the Roman people used to regard as a most honest and most wise judge, was in the habit of asking time and again in lawsuits: "to whom might it be for a benefit?"
|—Cicero: Pro Roscio Amerino, §§ 84, 86|