I though this was in the rules for downtime activities in either the PHB or the DMG, but I'm not seeing it.

Where are the rules for religious observations/rituals? I remember something along the lines of a gp value in sacrifices or expended materials, and a certain amount of time spent, led to Inspiration. Can anyone remember a page number or a search term I should be using?

Xanathar's Guide to Everything

Chapter 2: Downtime Revisited > Example Downtime Activities > Religious Service

Wasn't there something in the core rules though? I've not read Xanathar's.

There was something in the DMG

It's in the Downtime Activities section

Performing Sacred Rites

A pious character can spend time between adventures performing sacred rites in a temple affiliated with a god he or she reveres. Between rites, the character spends time in meditation and prayer.

A character who is a priest in the temple can lead these rites, which might include weddings, funerals, and ordinations. A layperson can offer sacrifices in a temple or assist a priest with a rite.

A character who spends at least 10 days performing sacred rites gains inspiration (described in chapter 4 of the Player’s Handbook) at the start of each day for the next 2d6 days.

That's the one I was looking for, thanks. It's weird that it doesn't come up under a search. 

Wow.  Thanks.  I'm pretty sure nobody in my party knows about this; inspiration for 2d6 days would be something some of them would want to jump on.