Reference for Shevuot 48:2

אמר רבא מאי טעמא דמאן דאית ליה איסור כולל

Raba said: What is the reason of the one who holds an inclusive prohibition [can take effect on a previous prohibition]? Because it is analogous to an extensive prohibition. And [the reason of] the one who exempts him, not holding this? Because he says, an extensive prohibition is applicable only to one piece, but not to two pieces. <br>

Jastrow