Thereafter the power of protection was generated from the arms of the gigantic virāṭ form, and in relation to such power the kṣatriyas also came into existence by following the kṣatriya principle of protecting society from the disturbance of thieves and miscreants.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
बाहुभ्योऽवर्तत क्षत्रं क्षत्रियस्तदनुव्रतः । यो जातस्त्रायते वर्णान् पौरुषः कण्टकक्षतात्
bāhubhyo'vartata kṣatraṃ kṣatriyastadanuvrataḥ | yo jātastrāyate varṇān pauruṣaḥ kaṇṭakakṣatāt
SB 3.6.31