Psalms 86:11

הוֹרֵנִי ָה’ דַּרְכֶּךָ אֲהַלֵּךְ בַּאֲמִתֶּךָ יַחֵד לְבָבִי לְיִרְאָה שְׁמֶךָ

Teach me Your way, Hashem, so that I might walk in Your truth; make my heart one to fear Your Name.

The latter clause of this verse, about unifying our hearts in the service of G-d, is cited in the brachos before reciting the Shema. The Metzudas David explains that by “unify our hearts,” the Psalm means that we should come to serve G-d with a single purpose. Our service should not be conditional on any matter, nor should we do so with any reservations.