SB 8.16.46
payo-bhakso vratam idam
cared visnv-arcanadrtah
purvavaj juhuyad agnim
brahmanams capi bhojayet
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Worshiping Lord Visnu 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 brahmanas as mentioned before.