The brāhmaṇas represent His mouth, the kṣatriyas His arms, the vaiśyas His thighs, and the śūdras are born of His legs.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
पुरुषस्य मुखं ब्रह्म क्षत्रमेतस्य बाहवः । ऊर्वोर्वैश्यो भगवतः पद्भ्यां शूद्रो व्यजायत
puruṣasya mukhaṃ brahma kṣatrametasya bāhavaḥ | ūrvorvaiśyo bhagavataḥ padbhyāṃ śūdro vyajāyata
SB 2.5.37