Saṃyutta Nikāya 4
Connected Discourses with Mara
At Savatthi. Now on that occasion the Blessed One was instructing, exhorting, inspiring, and gladdening the bhikkhus with a Dhamma talk concerning the five aggregates subject to clinging. And those bhikkhus were listening to the Dhamma with eager ears, attending to it as a matter of vital concern, applying their whole minds to it.
Then it occurred to Mara the Evil One: “This ascetic Gotama is instructing, exhorting, inspiring, and gladdening the bhikkhus … who are applying their whole minds to it. Let me approach the ascetic Gotama in order to confound them.”
Now on that occasion a number of almsbowls had been put out in the open. Then Mara the Evil One manifested himself in the form of an ox and approached those almsbowls. Then one bhikkhu said to another: “Bhikkhu, bhikkhu! That ox may break the almsbowls.” When this was said, the Blessed One said to that bhikkhu: “That is not an ox, bhikkhu. That is Mara the Evil One, who has come here in order to confound you.”
Then Mara the Evil One … disappeared right there.