Valmiki Ramayana Bala Kanda Sarga 6 Shloka 19
Original Shlokaक्षत्रं ब्रह्ममुखं चासीद्वैश्या: क्षत्रमनुव्रता: । शूद्रास्स्वधर्मनिरतास्त्रीन्वर्णानुपचारिण: ।।1.6.19।।
1 source checked for this shloka
क्षत्रम् the kshatriya (warrior) caste, ब्रह्ममुखम् आसीत् was obedient to brahmins, वैश्या: merchants, क्षत्रम् warriors, अनुव्रता: followed, शूद्रा: sudras, त्रीन् three, वर्णान् castes, उपचारिण: assisted, स्वधर्मनिरता: fully occupied in their respective professions.
The kshatriyas were obedient to brahmins and vaisyas followed kshatriyas. Sudras assisted the three castes and all were fully occupied in their respective professions.