Verse 17.13
विधिहीनमसृष्टान्नं मन्त्रहीनमदक्षिणम् । श्रद्धाविरहितं यज्ञं तामसं परिचक्षते ॥ १७.१३ ॥
शास्त्र विधि से हीन, अन्नदान से रहित, बिना मन्त्रों के, बिना दक्षिणा और बिना श्रद्धा के किये जाने वाले यज्ञ को तामस यज्ञ कहते हैं|
Finally, My dear Devotee, the lowest type of sacrifice that exists is that which is performed by those that have darkened their inner-beings and are known to be of the dull, Tamas nature. When Tamas-nature people make sacrificial offerings, it is generally not in accordance with the holy rules of sacrifice, no gifts of pure food are distributed (Prashadam), no holy hymns are chanted, and the sacrifice is performed with no religious faith in Me whatsoever.