Musnad Ahmad 1034:73

Mu'awiyah ibn 'Amr narrated to us, Za'idah narrated to us, from Husayn, from Shaqiq, who said: I heard Hudhayfah say: "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."