This is a slight misquotation of a comment in a letter by the Protestant theologian Martin Luther. Luther was responding to the charge that he had mistranslated a passage in Paul's letter to the Romans when producing his German translation of The Bible. The complete comment is:
Sic volo, sic jubeo. Sit pro ratione voluntas. Lutherus ita vult, et ait se doctorem esse super omnes doctores in toto papatu.
Which is to say,Thus I wish, thus I command. Let my will stand for a reason. Luther wishes it so, and says that he is a teacher above all the teachers in the whole of the Papacy.
Sic volo, sic jubeo, sit pro ratione voluntas is, in turn, a slight misquotation of a line from the Satire VI of the Roman poet Juvenal (Decimus Junius Juvenalis). Juvenal wrote Hoc volo, "this I wish".