Srimad-Bhagavatam: Canto 8 - Chapter 1 - Verse 46
Sanskrit:
पयोभक्षो व्रतमिदं चरेद् विष्णवर्चनादृत: ।पूर्ववज्जुहुयादग्निं ब्राह्मणांश्चापि भोजयेत् ॥ ४६ ॥
ITRANS:
payo-bhakṣo vratam idaṁcared viṣṇv-arcanādṛtaḥpūrvavaj juhuyād agniṁbrāhmaṇāṁś cāpi bhojayet
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.