"You shall not bear any grudge"Leviticus 19:18.

We are not allowed to bear a grudge against another – even if we never act upon the grudge.

For example: David asks Isaac to lend him his scythe, and Isaac refuses. Next day, Isaac needs an axe, and asks David whether he can borrow his. "I will lend you my axe," David responds, "I am not like you who refused to lend me your scythe..."