Deuteronomy 22:7

למען ייטב לך וגו׳ THAT IT MAY BE WELL WITH THEE etc. — If in the case of an easy command which involves no monetary loss, Scripture states “Do this in order that it may be well with thee and thou mayest prolong thy days”, it follows à fortiori that this at least will be the grant of the reward for the fulfilment of commands which are more difficult to observe (Chullin 142a).

שַׁלֵּ֤חַ תְּשַׁלַּח֙ אֶת־הָאֵ֔ם וְאֶת־הַבָּנִ֖ים תִּֽקַּֽח־לָ֑ךְ לְמַ֙עַן֙ יִ֣יטַב לָ֔ךְ וְהַאֲרַכְתָּ֖ יָמִֽים׃ {ס}        

but thou shalt surely let the mother go, and take the young to thee; that it may be well with thee, and that thou mayst prolong thy days.