Which RFC?

Anyone know *which* RFC says a packets should be routed using the most specific route in a routing table, not (for instance) the first route in the table that matches, or, for instance, using a less specific route that has a better metric? This is so basic I hardly know where to find it, but I have a (for the time being anonymous) systems vendor who really prefers the way their kernel does it .... Oddly enough it doesn't work well with subnetting. Alex Bligh Xara Networks