The source is an Israeli Microsoft employee describing a prank he pulled on his children on Passover.

Well, as you may know for Passover Seder, we set an extra place at the table for the Profit Elijah. The tradition normally is that we pour a cup of wine for the profit and the husband of the household open the door for him. Of course, the seat remains empty and the wine full. And many parents know you can have some fun with young children, who of course are watching the Elijah’s Cup intently, by knocking the table when they’re not looking so that some of the wine spills out and it appear that the cup is drunk. When they’re older maybe they don’t fall for this.

Anyway, last year we had the idea to take that one step further and I asked my friend from work Farhan to help me with a prank. He’s Zoroastrian so he’s not doing anything that night. So this Seder we set a place for Elijah like normal; we pour the wine like normal. My children are nine and thirteen so they don’t take the whole thing too seriously anymore; they know the trick of knocking the table and spilling the wine; you know, they’re too wise to fall for that anymore.

Well this year we start eating and suddenly a bearded olive-skinned man in a tunic walks in the front door, comes to Elijah’s place, drinks the wine, and walks out again without saying anything. My kids drop to the floor and they say, “who was that, Dad.”

And I say very casually, “That’s Eliyahu [Elijah].”

To this day I won’t tell them that it was really my friend Farhan.