The reason for Imam Sajjad (a.s) being given the title of Zainul ‘abidin was that one night, as he stood for prayers in his place of worship, Satan manifested himself in the form of a huge serpent with the intention of distracting him (a.s) from his prayers. But when the Imam (a.s) took no notice of him, the Satan approached him (a.s) and bit his big toe causing pain to run through him, but despite this act of his, the Imam (a.s) continued to remain oblivious of him and carried on with his prayers.
When, after the completion of his (a.s) prayers, he realized that it was Satan, he cursed him saying: “O’ Accursed! Go away,” and once again engaged himself in worship. It was at this moment that he (a.s) heard an angel call out three times: “You are the embellishment of the worshippers.”( Muntahal A’mal, vol. 2, pg. 3 )