Srimad Bhagavatam Canto 09, Chapter 06, Text 08

SB 9.6.8

sesam nivedayam asa
 pitre tena ca tad-guruh
coditah proksanayaha
 dustam etad akarmakam
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
Vikuksi offered the remnants of the flesh to King Iksvaku, who gave it to Vasistha for purification. But Vasistha could immediately understand that part of the flesh had already been taken by Vikuksi, and therefore he said that it was unfit to be used in the sraddha ceremony.
 
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
That which is meant to be offered in yajña cannot be tasted by anyone before being offered to the Deity. In our temples, this regulation is in effect. One cannot eat food from the kitchen unless it is offered to the Deity. If something is taken before being offered to the Deity, the entire preparation is polluted and can no longer be offered. Those engaged in Deity worship must know this very well so that they may be saved from committing offenses in Deity worship.
Srimad Bhagavatam Canto 09, Chapter 06, Text 07
Srimad Bhagavatam Canto 09, Chapter 06, Text 09