Musnad Ahmad 1034:125

Abu Mu'awiyah and Ibn Numayr narrated to us, Al-A'mash narrated to us, from Shaqiq, from Hudhayfah, who said: "When the Messenger of Allah, may Allah bless him and grant him peace, would rise at night, he would clean his mouth." Ibn Numayr said: I said to Al-A'mash: "With the siwak?" He said: "Yes."