Musnad Abu Ya'la al-Mawsili 154:119

Abu Abdullah ibn al-Dawraqi narrated to us, Abu Dawud narrated to us, Muhammad ibn Mihran al-Qurashi narrated to us, he said: My grandfather narrated to me, from Ibn Umar, that the Messenger of Allah (may Allah bless him and grant him peace) would not sleep except that the siwak was with him. When he would wake up, he would begin with the siwak.