Musnad Ahmad 1034:174

Waki’ narrated to us, Sufyan and ‘Abd al-Rahman narrated to us, from Sufyan, from Mansur and Husayn, from Abu Wa’il—‘Abd al-Rahman said: and al-A’mash—from Abu Wa’il, from Hudhayfah, who said: "When the Messenger of Allah (peace and blessings of Allah be upon him) would rise at night—and Waki’ said: for the Tahajjud—he would clean his mouth with the siwak."