Atithi Devo Bhava (अतिथिदेवो भव:)
Atithi Devo Bhava means “considers Guest as equivalent to God”
The source of mantra is Taittiriya Upanishad, Shikshavalli 11.2
Complete Mantra :
मातृदेवो भव।पितृदेवो भव। आचार्यदेवो भव। अतिथिदेवो भव !!
Meaning – “be one for whom the Mother is God, be one for whom the Father is God, be one for whom the Teacher is God, be one for whom the guest is God”.