"Athithi dévo bhava" is a part of Taittirīya (deals with 'from food to joy') Upanishad "Mătru dévo bhava, Pitru dévo bhava, Ăchărya dévo bhava, Athithi dévo bhava". Means Mother, Father, Guru/Teacher and Guests (athithi) are divine, should be revered as God.