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