John Wesley on 1 Samuel 15:22

Sacrifice - Because obedience to God is a moral duty, constantly and indispensably necessary; but sacrifice is but a ceremonial institution, sometimes unnecessary, as it was in the wilderness: and sometimes sinful, when it is offered by a polluted hand, or in an irregular manner. Therefore thy gross disobedience to God's express command, is not to be compensated with sacrifice. Hearken - That is, to obey. Fat - Then the choicest part of all the sacrifice.