As per Wikipedia article on bar bets (... from the "last places you'd think to find things" category :)
It is widely believed that the creation of Scientology was the result of a bar bet between L. Ron Hubbard and Robert A. Heinlein. The story says L. Ron Hubbard dared that he could create a religion all by himself. According to Scientology critic Lindsay this is "definitely not true", no such bet was ever made, it would have been "uncharacteristic of Heinlein" to make such a bet, and "there's no supporting evidence". However, several of Heinlein's autobiographical pieces, as well as biographical pieces written by his wife, claim repeatedly that the bet did indeed occur.
Please note that the "biographical pieces" does NOT refer to the main RAH "biographical" book, "Grumbles from the Grave" - a full text search found no mentions of 'bar', 'bet', 'religion', 'hubbard' or 'dianetics' in the desired context.
The only other place it can be is a bunch of letters in RAH/VH archives, but those cost a lot of money to access online ($3-$12 per archive item) and as such I'm afraid I won't be able to confirm/deny using those; but I included the URL for those who may want to.