Chapter 17 · verse 13 · gita.yoga.org

Bhagavad Gita 17.13

विधिहीनमसृष्टान्नं मन्त्रहीनमदक्षिणम्।श्रद्धाविरहितं यज्ञं तामसं परिचक्षते

They declare that sacrifice to be Tamasic which is contrary to the ordinances of the scriptures, in which no food is distributed, and which is devoid of mantras, gifts, and faith.