Sunan an-Nasa'i, Volume 2, Book 20, Hadith 1625 (DarusSalam) ⓘ
It was narrated that Shaqiq said: "We were commanded, when we got up to pray at night, to clean our mouths with the siwak."
أَخْبَرَنَا أَحْمَدُ بْنُ سُلَيْمَانَ، قَالَ حَدَّثَنَا عُبَيْدُ اللَّهِ، قَالَ أَنْبَأَنَا إِسْرَائِيلُ، عَنْ أَبِي حَصِينٍ، عَنْ شَقِيقٍ، قَالَ كُنَّا نُؤْمَرُ إِذَا قُمْنَا مِنَ اللَّيْلِ أَنْ نَشُوصَ أَفْوَاهَنَا بِالسِّوَاكِ .
also referenced under: Hadith 1624 (Reference) & Book 20, Hadith 27 (In-Book)