Not every authentic hadith is confined to the Two Ṣaḥīḥs.
Ṣaḥīḥ al-Bukhārī and Ṣaḥīḥ Muslim do contain a large body of rigorously sound reports, yet what they include is by no means the sum total of authentic hadith. Just as sound narrations appear in their two collections, they are also found elsewhere in works devoted to the Prophetic tradition such as al-Muwaṭṭaʾ, the Ṣaḥīḥ of Ibn Khuzaymah, the Ṣaḥīḥ of Ibn Ḥibbān, al-Ḥākim’s al-Mustadrak, al-Tirmidhī’s Jāmiʿ, the Sunan of Abū Dāwūd, al-Nasāʾī, Ibn Mājah, al-Dārimī, al-Dāraqutnī, al-Bayhaqī, and others besides. This is perfectly evident, for nowhere do we find al-Bukhārī or Muslim claiming that their books encompassed all authentic material or that they even intended such comprehensiveness. On the contrary, each of them stated explicitly otherwise.
Abū ʿAmr Ibn al-Ṣalāḥ says in his ʿUlūm al-Ḥadīth: “They—meaning al-Bukhārī and Muslim—did not gather every authentic report in their two Ṣaḥīḥs, nor did they bind themselves to that. We have it on record that al-Bukhārī said: ‘I have included in my Jāmiʿ only what is sound, and I have left out much that is sound for fear of excessive length.’ We likewise have it on record that Muslim said: ‘Not everything I consider authentic has been placed here—in other words, in my Ṣaḥīḥ. I have placed here only that upon which the scholars are unanimously agreed …’”
Al-Ḥāfiẓ Ibn Ḥajar, in the introduction to Fatḥ al-Bārī, relates from al-Ismāʿīlī that al-Bukhārī said: “I did not include in this book except what is authentic, and what I have left out of authentic material is even more.”
Al-Nawawī, in the preface to his commentary on Ṣaḥīḥ Muslim, after noting that some scholars sought to oblige the two Imams to include additional reports that meet their criteria but which they had omitted, remarks: “In reality this obligation does not stand, for the two never undertook to gather every authentic report. It is authentically reported from each of them that they did not attempt such comprehensiveness; rather, their purpose was to assemble representative portions of what is sound, as…"