According to the notes in Plato: Republic Book X, edited by John Ferguson, p. 71, «the familiar 'amicus Plato sed magis amica veritas' is found in Cervantes' Don Quixote II 8 and cannot be traced further back. Cf. Roger Bacon Op. mai. I vii, 'amicus est Socrates, magister meus, sed magis est amica veritas'. For the opposite view, see Cicero, T.D. I 17,39, 'errare mehercule malo cum Platone . . . quam cum istis vera sentire'.»https://en.wikiquote.org/wiki/Aristotle