Spartans, children of Abraham?

OK, I am a bit perplexed and I need some clarification. In 1 Macc. 12.1-23 the Jews are solidifying their allegiance with the Romans and they send a letter to the Spartans asking them to join in “brotherhood and friendship” (v. 10). As I read this I assumed this to be the famous Spartans, but one statement confused me.

“A document has been found stating that the Spartans and the Jews are brothers; both nations descended from Abraham.” (v. 21)

Huh? I have never heard such an odd claim. I continue to assume that these are the famous Spartans, but from where in the world did the connection to the Patriarch Abraham come? Is this merely a rhetorical move to bring solidarity with the Jews? Is there any other place in ancient literature where anything similar to this statement can be found?