For R. Jannai said: Tefillin demand a pure body, like Elisha-the-man-of-the-wings. What does this mean? — Abaye said: That one must not pass wind while wearing them; Raba said: That one must not sleep in them. And why is he called 'the man-of-the-wings'? Because the wicked State once proclaimed a decree against Israel that whoever donned tefillin should have his brains pierced through; yet Elisha put them on and went out into the streets. A quaestor saw him: he fled before him, and the latter gave pursuit. As he overtook him, he [Elisha] removed them from his head and held them in his hand, 'What is that in your hand?' he demanded, 'The wings of a dove,' was his reply. He stretched out his hand and the wings of a dove were found therein. Hence he is called 'Elisha-the-man-of-the-wings.'

And why did he tell him the wings of a dove rather than that of other birds? Because the Congregation of Israel is likened to a dove, as it is said, as the wings of a dove covered with silver, and her pinions with yellow gold:  just as a dove is protected by its wings, so with the Israelites, their precepts protect them.