SB 8.16.46

payo-bhakṣo vratam idaṁ
 cared viṣṇv-arcanādṛtaḥ
pūrvavaj juhuyād agniṁ
 brāhmaṇāṁś cāpi bhojayet
Word for word: 
payaḥ-bhakṣaḥ — one who drinks milk only; vratam idam — this process of worshiping with a vow; caret — one should execute; viṣṇu-arcana-ādṛtaḥ — worshiping Lord Viṣṇu with great faith and devotion; pūrva-vat — as prescribed previously; juhuyāt — one should offer oblations; agnim — into the fire; brāhmaṇān — unto the brāhmaṇas; ca api — as well as; bhojayet — should feed.
Translation: 
Worshiping Lord Viṣṇu with great faith and devotion and living only by drinking milk, one should follow this vow. One should also offer oblations to the fire and feed the brāhmaṇas as mentioned before.