From the Super Mario Wiki
In The Adventures of Super Mario Bros. 3 episode "Mind Your Mummy Mommy, Mario", Queen Mushroomkhamen mistook Mario for her son, and took her back to her tomb. Not knowing how to find their way through the tombs of Desert Land's pyramids, Princess Toadstool commented that they needed a map of the pyramids if they were ever going to find Mario.
The mention of maps reminded Toad of his cousin, and he brought Princess Toadstool and Luigi to see his cousin. Wanting to make several sales, Toad's cousin advertised all sorts of maps to Princess Toadstool, including maps to the pyramids, mummy tombs, and even the houses of Mushroom celebrities. In the end, he simply sold a map to the mummy tombs to Princess Toadstool.
Although Toad's cousin was never seen again, Luigi announced that he needed to find him again, so that he could buy a map to "someplace else", due to the fact that Queen Mushroomkhamen had mistaken him for her husband, and was about to chase him throughout Desert Land.