When the legislative and executive powers are united in the same person, or in the same body of politicians. Which philosopher would have been most likely to say this quote?

That statement was made by Baron de Montesquieu.

When the legislative and executive powers are united in the same person, or in the same body of magistrates, there can be no liberty; because apprehensions may arise, lest the same monarch or senate should enact tyrannical laws, to execute them in a tyrannical manner.