Blessing Within The Tochacha
בָּנִים וּבָנוֹת, תּוֹלִיד; וְלֹא-יִהְיוּ לָךְ, כִּי יֵלְכוּ בַּשֶּׁבִי
You will bear sons and daughters, but they will not be yours, for they will go into captivity. (Devarim 28:41)
Degel Machaneh Ephraim, Parshas Ki Savo:
This is also a great blessing. Mitzvos and good deeds are also referred to as בָּנִים (children). When the verse says "they will not be yours", this means that you will do these mitzvos without the ulterior motive of receiving a reward. Rather, your sole intention will be to return the captive fallen sparks back to their source.