Musnad Ahmad 27:1520

Sulayman ibn Dawud narrated to us, Muhammad ibn Muslim ibn Mihran, a freed slave of Quraysh, narrated to us, saying: I heard my grandfather narrating from Ibn Umar, "That the Messenger of Allah (peace and blessings of Allah be upon him) would not sleep except that the siwak was with him. So when he awoke, he would begin with the siwak."