User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-5247339-20130520192736/@comment-3146930-20130524215050

Grover mentions the oath in Chapter 8 of The Lightning Thief.

"The winning side, Zeus and Poseidon, made Hades swear an oath with them: no more affairs with mortal women. They all swore on the River Styx."