It's not in the ELH, or the Errata (that I can see). I'm not seeing it the PHB or DMG from the 2012 reprint either.
It doesn't seem like it's just a logical extension of the rules applied to any given prestige class with 10 levels as there is the curious alternating progression for spells/day on odd/even levels ... so where did it come from? Am I blind? Or does this curiosity only exist in the 3.5 PHB/DMG and not in the 2012 reprints?