Bodhidharma sat facing a wall.
The Second Patriarch, standing in the snow, stood behind him and cut off his arm. He yelled, "My mind is not yet at peace. I beg you, master, put my mind to rest!"
Bodhidharma didn't turn around. He said, "Bring your mind to me, and I will put it to rest."
The Second Patriarch stayed up all night, and all the next day looking for his mind. The next evening he returned to his master and said, "I have searched for it and searched for it, but I cannot find my mind anywhere."
Bodhidharma said, "I have finished putting it to rest for you."
— Mumon, The Gateless Gate, Case 41: Bodhidharma Puts the Mind to Rest, China, 13th Century C.E.