The means of livelihood of all persons, namely production of grains and their distribution to the prajās, was generated from the thighs of the Lord’s gigantic form. The mercantile men who take charge of such execution are called vaiśyas.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
विशोऽवर्तन्त तस्योर्वोर्लोकवृत्तिकरीर्विभोः । वैश्यस्तदुद्भवो वार्तां नृणां यः समवर्तयत्
viśo'vartanta tasyorvorlokavṛttikarīrvibhoḥ | vaiśyastadudbhavo vārtāṃ nṛṇāṃ yaḥ samavartayat
SB 3.6.32