Goscinny

Cleopatra promises Caesar that she will build him a palace within three months to prove Egypt's strength. Her architect Edifis calls on his old friends Getafix, Asterix and Obelix, who go to Egypt and help the Queen win her bet with Caesar.

All of the Asterix stories feature references to Obelix falling into the Druid's Cauldron of magic potion when he was about six years old. This was the event through which he derived his phenomenal physical strength. Here is the full story of how it happened.