That's a joke that almost everyone knows, yet nobody knows where it came from. Sounds like another challenge for the Oracle at Google. Most of the hits were just the joke being repeated with variations, but I finally found a hit that cited a source:

The story is "Answer," from Angels and Spaceships, by Fredric Brown (Dutton, 1954). Here is the original text:

Dwar Ev ceremoniously soldered the final connection with gold. The eyes of a dozen television cameras watched him and the subether bore through the universe a dozen pictures of what he was doing.

He straightened and nodded to Dwar Reyn, then moved to a position beside the switch that would complete the contact when he threw it. The switch that would connect, all at once, all of the monster computing machines of all the populated planets in the universe--ninety-six billion planets--into the supercircuit that would connect them all into the one supercalculator, one cybernetics machine that would combine all the knowledge of all the galaxies.

Dwar Reyn spoke briefly to the watching and listening trillions. Then, after a moment's silence, he said, "Now, Dwar Ev."

Dwar Ev threw the switch. There was a mighty hum, the surge of power from ninety-six billion planets. Lights flashed and quieted along the miles-long panel.

Dwar Ev stepped back and drew a deep breath. "The honor of asking the first question is yours, Dwar Reyn."

"Thank you," said Dwar Reyn. "It shall be a question that no single cybernetics machine has been able to answer."

He turned to face the machine. "Is there a God?"

The mighty voice answered without hesitation, without the clicking of single relay.

"Yes, now there is a God."

Sudden fear flashed on the face of Dwar Ev. He leaped to grab the switch.

A bolt of lightning from the cloudless sky struck him down and fused the switch shut.