"Descartes's original statement was "Je pense donc je suis," from his Discourse on Method (1637). He wrote it in French, not in Latin, thereby reaching a wider audience in his country than that of scholars. He uses the Latin "Cogito ergo sum" in the later Principles of Philosophy (1644), Part 1, article 7: "Ac proinde hæc cognitio, ego cogito, ergo sum, est omnium prima & certissima, quæ cuilibet ordine philosophanti occurrat." (English: "This proposition, I think, therefore I am, is the first and the most certain which presents itself to whomever conducts his thoughts in order.")."

"The phrase Cogito ergo sum is not used in Descartes' Meditations on First Philosophy, but the term "the cogito" is (often confusingly) used to refer to an argument from it. Descartes felt that this phrase, which he had used in his earlier Discourse, had been misleading in its implication that he was appealing to an inference, so he avoided the word ergo and wrote "that the proposition, I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind." (Meditation II.)"

(Wiki)