It is narrated from Sayyiduna Hudhayfah (may Allah be pleased with him) that when the Messenger of Allah (peace and blessings be upon him) woke at night, he would clean his mouth with the siwak.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 163
Hushaym bin Bashir narrated it from Husayn, and said in the hadith that when the Messenger of Allah (peace and blessings be upon him) stood for the Tahajjud prayer, he would clean his mouth with the siwak.
(b) Abu Sulayman al-Khattabi (may Allah have mercy on him) said: "Shaws" means rubbing the teeth with the siwak, the finger and the like.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 164
(a) It is narrated from the Mother of the Believers, Sayyidah Aishah (may Allah be pleased with her), that we used to keep the siwak and the water for ablution ready for the Messenger of Allah (peace and blessings be upon him). Whenever Allah Almighty willed to wake him (peace and blessings be upon him) at night, he (peace and blessings be upon him) would use the siwak and perform ablution, and then pray.
(b) In the narration of Bahz bin Hakim from Zurarah (may Allah have mercy on him), his water for ablution and his siwak used to be placed for him (peace and blessings be upon him); he (peace and blessings be upon him) would get up at night, relieve himself, and then use the siwak.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 165
It is narrated from the Mother of the Believers, Sayyidah Aishah (may Allah be pleased with her), that whenever the Prophet (peace and blessings be upon him) woke up after sleeping, by day or by night, he would use the siwak before performing ablution.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 167