Reference for Gittin 105:14

אי הכי אפילו במזיד נמי השתא לאוזוקי קא מכוין אודועי לא מודע ליה

R. Johanan said that the rule of the Torah is that whether one commits these offences innocently or deliberately he is not liable, the reason being that damage of which there is no visible sign is not legally accounted damage. Why then did the Rabbis ordain that [one who does them] presumptuously is liable? So that it should not become a common thing for a man to go and render unclean the foodstuffs of his neighbour and say, I have no liability.

Jastrow