Reference for Chullin 281:18

שלח אפילו מאה פעמים תשלח אין לי אלא לדבר הרשות לדבר מצוה מנין

R'Abba the son of R'Joseph B'Raba said to R'Kahana: Then the only reason [for this] is that the Divine Law stated 'teshallah', but otherwise I should have said that [where one required the dam] for the fulfilment of a precept, the law did not apply.