Chandyoga Upanisad

“It is like this. Take a bird that is tied with a string. It will fly off in every direction and, when it cannot find a resting-place anywhere else, it will alight back upon the very thing to which it is tied. Similarly, son, the mind flies off in every direction and, when it cannot find a resting-place anywhere else, it alights back upon the breath itself; for the mind, my son, is tied to the Breath."

 Chapter 6 #8