The Buddha explains to Sutavā, the wanderer, that an arahant is incapable of transgressing in nine ways.
Sutavā sutta - To Sutavā
At one time, the Blessed One was dwelling in Rajagaha at Vulture Peak Mountain. Then Sutavā, the wanderer, approached the Blessed One; having approached, he exchanged greetings with the Blessed One. After having exchanged pleasant and polite conversation, he sat down to one side. After conversing, he sat down to one side and said to the Blessed One:
"Venerable sir, one time, while dwelling here in Rājagaha at the mountain cave, I directly heard and learned from the Blessed One: 'He who, Sutavā, is a bhikkhu who is an arahant, whose defilements are destroyed, who has lived the spiritual life, done what was to be done, laid down the burden, reached the ultimate goal, destroyed the fetters of existence, and is released by perfect wisdom, he is incapable of transgressing in five ways — incapable of intentionally taking the life of a living being, incapable of taking what is not given, incapable of engaging in sexual intercourse, incapable to knowingly speak falsely, and incapable to use stored up goods as he did before, when he was a householder. Is it true, venerable sir, that this was well heard, well learned, well attended to, and well retained by me?"
"Indeed, Sutavā, this was well heard, well learned, well attended to, and well retained by you. Before and now, I declare: 'A bhikkhu who is an arahant, whose defilements are destroyed, who has lived the spiritual life, done what was to be done, laid down the burden, reached the ultimate goal, destroyed the fetters of existence, and is released by perfect wisdom, is incapable of transgressing in nine ways — incapable of intentionally take the life of a living being, incapable of taking what is not given, incapable of engaging in sexual intercourse, incapable to knowingly speak falsely, incapable to use stored-up goods as he did before, when he was a householder, incapable to act out of impulse, incapable to act out of aversion, incapable to act out of delusion, and incapable to act out of fear.' Before and now, I declare: 'He who is a bhikkhu who is an arahant, whose defilements are destroyed, who has lived the spiritual life, done what was to be done, laid down the burden, reached the ultimate goal, destroyed the fetters of existence, and is released by perfect wisdom is incapable of transgressing in these nine ways.'"