Musannaf Ibn Abi Shayba 1:1783

Abu Bakr narrated to us, he said: Hushaym narrated to us, from Husayn, from Abu Wa'il, from Hudhayfah ibn al-Yaman, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) would stand to perform the night prayer (tahajjud), he would scrub his mouth with the siwak.