"My father told me: 'I heard 'Aishah say, (narrating) from the Prophet (ﷺ): "Siwak is a means of purification for the mouth and is pleasing to the Lord."
Sunan an-Nasa'i Book 1, Hadith 5
← Previous Hadith "I came to the Prophet (ﷺ) when he was using the Siwak and with me were two men of the Ash'aris - one on my right and the other on my left - who were seeking to be appointed as officials. I said: 'By the One Who sent you as a Prophet with the truth, they did not tell me why they wanted to come with me and I did not realize that they were seeking to be appointed as officials.' And I could see his Siwak beneath his lip, then it slipped and he said: 'We do not' - or; 'We will never appoint as an official anyone who seeks that. Rather you should go.'" So he sent his (Abu Musa) to Yemen, then he sent Mu'adh bin Jabal to go after him - may Allah be pleased with them.

Sunan an-Nasa'i Book 1, Hadith 4 Next Hadith → "The Messenger of Allah (ﷺ) said: 'I have indeed urged you with regard to the Siwak.'"

Sunan an-Nasa'i Book 1, Hadith 6