“If you believe in yourself, anything is possible.”

The Mask of Apollo (1966)

Source: Oranges Are Not the Only Fruit

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Attributed

Attributed to Korda in The Power of Choice (2007) by Joyce Guccione, p. 52, the earliest occurrence of such phrasing yet located is by Martin Lawrence, in "What Up?" in Upscale : The Successful Black Magazine (February 1993), p. 79: "If you don't believe in yourself, then who will believe in you? The next man's way of getting there might not necessarily work for me, so I have to create my own ways of getting there."
Disputed

אם אתה מאמין שיכולים לקלקל, תאמין שיכולים לתקן Im ata ma'amin sh'ykholim lekalkel, ta'amin sh'yecholim letaken.
Attributed

In reply to a man who greeted him in the street with the words "Mr. Jones, I believe?", as quoted in Wellington — The Years of the Sword (1969) by Elizabeth Longford.

Source: Everybody’s Autobiography (1937), Ch. 2