Charles H. Spurgeon on Psalms 31:19

Very observable is that expression of the psalmist, Oh how great is thy goodness which thou hast laid up for them that fear thee; which thou hast wrought before the sons of men for them that trust in thee. In the former clause, God's goodness is said to be laid up; in the latter, to be wrought. Goodness is laid up in the promise, wrought in the performance; and that goodness which is laid up is wrought for them that trust in God; and thus, as God's faithfulness engages us to believe, so our faith, as it were, engages God's faithfulness to perform the promise. Nathanael Hardy.