Sahih Muslim 253a
Sahih Muslim · Book 2: The Book of Purification · Siwak (tooth-stick)
Miqdam b. Shuraih narrated it from his father who said:
حَدَّثَنَا أَبُو كُرَيْبٍ، مُحَمَّدُ بْنُ الْعَلاَءِ حَدَّثَنَا ابْنُ بِشْرٍ ، عَنْ مِسْعَرٍ ، عَنِ الْمِقْدَامِ بْنِ شُرَيْحٍ ، عَنْ أَبِيهِ ، قَالَ سَأَلْتُ عَائِشَةَ قُلْتُ بِأَىِّ شَىْءٍ كَانَ يَبْدَأُ النَّبِيُّ صلى الله عليه وسلم إِذَا دَخَلَ بَيْتَهُ قَالَتْ بِالسِّوَاكِ .
I asked A'isha what Allah's Apostle (ﷺ) did first when he entered his house, and she replied: He used tooth-stick (first of all).