Jaiminīya Upaniṣad Brāhmaṇa 2.15.2

2.15.2

tan na vratyam adadāno 'śnīyāt
yo vai mahāśane 'naśnaty aśnātīśvāro hainam abhiṣaṅktoḥ
pūtim iva hāśnīyāt /

Therefore, one should not eat without having given him his vow-portion. He who eats while the great eater is not eating, [the fire] is capable of seizing him; he would eat as if it were putrid.