There are no explicit rules just for this one thing. Instead it falls under a broader umbrella. Check out the DMG, p.42. Actions the Rules Don't Cover, Other Checks: If the action is related to a skill .. use that check. If it is not an obvious skill or attack roll, use an ability check. Consult the .. table .. and set the DC according to .. the task. For example, you rule that tossing a potion to someone 1 or 2 squares away is Easy, 3-4 squares away is Medium and 5+ squares away (maximum 6, say), is Hard and you rule that the appropriate check is a DEX check. You could also rule that the person catching it has to be able to take opportunity actions, i.e., not Dazed, Stunned, etc., and must have a hand free. You could rule further that the person catching has to make a check of some kind of their own, like an Easy DEX check. If either roll fails, the potion hits the floor with a splash. Or does the fallen potion get a saving throw? Again, there is no rule stating what happens to dropped potions, so that's up to you! You could even invoke a 'scatter' roll to see what square the botched throw ends up.Is any or all of this a true house rule, or is this just an interpretation of how to use the Actions the Rules Don't Cover rule? That's up to you, but I daresay that when things come up like this in an official event, the players will expect some kind of ruling as opposed to 'you can't do that there are no rules for it!'
I'm trying to find out if there are any rules dealing with tossing items that can be held in one hand to another ally.