This one is commonly attributed to the Buddha, although it’s very modern and law-of-attraction-y. It’s also found as “What you think you become, what you feel you attract, what you imagine you create.”
I believe that the quote in this form was written by Adele Basheer, who designs greetings cards for her company, Intrinsic.
According to her website, “While her personal mantra is the “what you think you create” message, Adele also feels that all it takes is believing…”
Basheer has taken these phrases from the writings of the Irish-born New Thought author Joseph Murphy. In his 1963 book “The Power of Your Subconscious Mind” he said, “Know that belief is a thought in your mind, and what you think you create.” And in his 1968 “The Cosmic Power Within You” he wrote, “You have faith when you know that thoughts are things; what you feel, you attract; and what you imagine, you become.”
Sometimes the quote is prefaced by “The law of the mind is relentless…” but I’ve found no source for that. If that’s by Murphy then I haven’t yet found it in any of his books.
The Buddha, on the other hand, pointed out in the Ittha Sutta that if we want qualities like long life, beauty, happiness, status, or a good rebirth, there’s no point simply wishing for those things. Wishing positive things is fine, but we must also engage with the path of practice that leads to them.
He certainly wouldn’t agree with a statement like “all it takes is believing,” and in fact that’s a notion that he roundly ridiculed:
Suppose a man were to throw a large boulder into a deep lake of water, and a great crowd of people, gathering and congregating, would pray, praise, and circumambulate with their hands palm-to-palm over the heart [saying,] ‘Rise up, O boulder! Come floating up, O boulder! Come float to the shore, O boulder!’ What do you think: would that boulder — because of the prayers, praise, and circumambulation of that great crowd of people — rise up, come floating up, or come float to the shore?
I can imagine that discourse raising a few chuckles from the bhikkhus!