Bhagavad Gita 17.13

विधिहीनमसृष्टान्नं मन्त्रहीनमदक्षिणम्। श्रद्धाविरहितं यज्ञं तामसं परिचक्षते

vidhi-hīnam asṛiṣhṭānnaṁ mantra-hīnam adakṣhiṇam śhraddhā-virahitaṁ yajñaṁ tāmasaṁ parichakṣhate

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.

शास्त्रविधिसे हीन, अन्न-दानसे रहित, बिना मन्त्रोंके, बिना दक्षिणाके और बिना श्रद्धाके किये जानेवाले यज्ञको तामस यज्ञ कहते हैं।