Ibn 'Abbas told that a nomadic Arab came to God's messenger and asked, "By what means can I know that you are a prophet?" He replied, "If I call this raceme from this palm-tree it will testify that I am God's messenger." He called it and it began to come down from the palm-tree and fell beside the Prophet. He then told it to go back, and when it returned the nomadic Arab accepted Islam.
Tirmidhi transmitted it, saying it is sahih.