Who told Benaiah to kill Adonijah?

Solomon

Now therefore, as the LORD liveth, which hath established me, and set me on the throne of David my father, and who hath made me an house, as he promised, Adonijah shall be put to death this day.

And king Solomon sent by the hand of Benaiah the son of Jehoiada; and he fell upon him that he died. 1 Kings 2:24-25