Source, in full
Widely attributed to Leo X, the earliest known source of this statement is actually a polemical work by the Protestant John Bale, the anti-Catholic Acta Romanorum Pontificum, which was first translated from Latin into English as The Pageant of the Popes in 1574: "For on a time when a cardinall Bembus did move a question out of the Gospell, the Pope gave him a very contemptuous answer saying: All ages can testifie enough how profitable that fable of Christe hath ben to us and our companie." The Pope in this case being Leo X.
Notes from Wikiquote
In a more modern polemic, "The Criminal History of the Papacy" by Tony Bushby, in Nexus Magazine Volume 14, Number 3 (April - May 2007), it is stated that "The pope's pronouncement is recorded in the diaries and records of both Pietro Cardinal Bembo (Letters and Comments on Pope Leo X, 1842 reprint) and Paolo Cardinal Giovio (De Vita Leonis Decimi..., op. cit.), two associates who were witnesses to it."