As per the māntra mentioned in the Tāittiriya Brāhmana, the Kshātriya originates from the Brāhmana.
ब्रह्मणः क्षत्रं निर्मितम् ब्रह्म ब्राह्मण आत्मा, इति.
ब्रह्मणः सकाशात्क्षत्रं निर्मितं क्षत्रियजातिनि-मिता । यत्परं ब्रह्म तदात्मना स्वस्वरूपेणैत्र ब्राह्मणोऽभवत्
A similar statement is found in the Shatapatha Brāhmana,Verse 14.4.2.23 which states that Kshatra originated from Brahma, and that Brahma established itself within Kshatra.
brahma vā idamagra āsīt ekameva tadekaṃ sanna vyabhavattacreyo rūpamatyasṛjata kṣatraṃ yānyetāni devatrā kṣatrāṇīndro varuṇaḥ somo rudraḥ parjanyo yamo mṛtyurīśāna iti tasmātkṣatrātparaṃ nāsti tasmādbrāhmaṇaḥ k���atriyamadhastādupāste rājasūye kṣatra eva tadyaśo dadhāti saiṣā kṣatrasya yoniryadbrahma tasmādyadyapi rājā paramatāṃ gacati brahmaivāntata upaniśrayati svāṃ yoniṃ ya u enaṃ hinasti svāṃ sa yonimṛcati sa pāpīyānbhavati yathā śreyāṃsaṃ hiṃsitvā
This mantra is also present in Sage Yajnavalkya’s Brihadaranyaka Upanishad. सैषा क्षत्रस्य योनिर्यद्ब्रह्म
Furthermore, both the Gautama Dharmasutra and the Apastamba Dharmasutra identify the Brahmana as the progenitor of the Kshatriya.
brāhmaṇaṃ daśavarṣaṃ tu śatavarṣaṃ tu bhūmipam
Āpastamba-Dharmasūtra 1.14.25.
Mahābhārata 13.8.21.
Viṣṇu-Smṛti 32.17
Manusmriti Verse 2.135