Musnad Ahmad 1060:43

Muhammad ibn Ubayd narrated to us, Wasil narrated to us, from Abu Surah, from Abu Ayyub: "That the Messenger of Allah, may Allah bless him and grant him peace, used to use the siwak at night two or three times. And when he stood to pray at night, he would pray four rak'ahs, not speaking and not commanding anything, and he would perform the taslim between every two rak'ahs."