(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