I am a DM with a 2nd lvl priest. I want my priest to be able to cast a remove disease spell off of a scroll that he will find. Can he do this? Where is the rule?
Yes, by making a caster level check and meeting some requirements
The relevant rules are all in the Magic Items - Scrolls section of the DMG. Here's a copy from the SRD. There are two steps:
Decipher the Writing
The writing on a scroll must be deciphered before a character can use it or know exactly what spell it contains. This requires a read magic spell or a successful Spellcraft check (DC 20 + spell level).
Deciphering a scroll to determine its contents does not activate its magic unless it is a specially prepared cursed scroll. A character can decipher the writing on a scroll in advance so that he or she can proceed directly to the next step when the time comes to use the scroll.
Read Magic is a level 0 Cleric spell, so your player can prepare and cast it to perform this step. They could also use the Spellcraft skill instead, but a DC 23 Spellcraft check can be difficult for a level 2 Cleric to make.
Activate the Spell
To have any chance of activating a scroll spell, the scroll user must meet the following requirements.
The spell must be of the correct type (arcane or divine). Arcane spellcasters (wizards, sorcerers, and bards) can only use scrolls containing arcane spells, and divine spellcasters (clerics, druids, paladins, and rangers) can only use scrolls containing divine spells. (The type of scroll a character creates is also determined by his or her class.)
Assuming you're giving out a Divine scroll, this is no problem.
The user must have the spell on his or her class list.
Clerics have Remove Disease on their spell list, so this is no problem.
The user must have the requisite ability score.
Remove Disease is a third level Cleric spell, so your Cleric requires 13 Wisdom to meet this requirement.
If the user meets all the requirements noted above, and her caster level is at least equal to the spell’s caster level, she can automatically activate the spell without a check. If she meets all three requirements but her own caster level is lower than the scroll spell’s caster level, then she has to make a caster level check (DC = scroll’s caster level + 1) to cast the spell successfully. If she fails, she must make a DC 5 Wisdom check to avoid a mishap (see Scroll Mishaps, below). A natural roll of 1 always fails, whatever the modifiers.
Being a third level spell, the caster level requirement is 5. You can tell that because it's a third level spell, and Clerics get third level spells at level 5.
Your Cleric is likely a level 2 Cleric, thus has a caster level of 2. That isn't sufficient to meet this requirement, so your Cleric will have to make a caster level check of DC 6 (5 + 1). A caster level check is 1d20 + caster level, or 1d20+2. Only needing a a total of 6, a 4 or better on the dice will let your Cleric cast the spell. Those are good odds.
At Cleric level 5, your player can cast the scroll without having to make the check.
If your Cleric doesn't meet the other requirements, or someone else wants to use the scroll, they have to use Use Magic Device instead.