Musnad Ahmad 1034:217

Muhammad ibn Ja‘far narrated to us, he said: Shu‘bah narrated to me, from Husayn, who said: I heard Abu Wa’il narrating from Hudhayfah, who said: "When the Messenger of Allah, peace and blessings of Allah be upon him, would stand for the Tahajjud (night prayer), he would clean his mouth with the siwak."