Manusmriti
यो न वेत्त्यभिवादस्य विप्रः प्रत्यभिवादनम् । नाभिवाद्यः स विदुषा यथा शूद्रस्तथैव सः ॥ १२६ ॥
A Brahmana who does not know the form of returning a salutation, must not be saluted by a learned man; as a Sudra, even so is he.
english translation
yo na vettyabhivAdasya vipraH pratyabhivAdanam । nAbhivAdyaH sa viduSA yathA zUdrastathaiva saH ॥ 126 ॥
hk transliteration by Sanscript