We are what we think. All that we are arises with our thoughts. With our thoughts we make the world. Speak or act with an impure mind and trouble will follow you as the wheel follows the ox that draws the cart… Speak or act with a pure mind and happiness will follow you as your shadow, unshakable.
Source: The Dhammapada: The Sayings of the Buddha, trans. Thomas Byrom, 1976 (verses 1-2)
This is Dhammapada 1-2 (Yamakavagga) in Thomas Byrom's 1976 rendering, which is the source of the familiar 'We are what we think' opening. Byrom is a free poetic translator: more literal versions render the opening as 'Mind precedes all mental states; mind is their chief.' So the underlying text is canonical and the ox-cart and shadow similes are genuine, but the English is Byrom's interpretation rather than a close translation. Attribution to 'Buddha' is correct in the traditional sense.