Verse
विधिहीनमसृष्टान्नं मन्त्रहीनमदक्षिणम् ।श्रद्धाविरहितं यज्ञं तामसं परिचक्षते ॥
vidhihīnamasṛṣṭānnaṃ mantrahīnamadakṣiṇamśraddhāvirahitaṃ yajñaṃ tāmasaṃ paricakṣate
The sacrifice not performed according to the right rule of the Shastra, without giving of food, without the mantra, without gifts, empty of faith, is said to be tamasic.
Gītā 17·13