Cast your bread on the waters; For you shall find it after many days. — Ecclesiastes 11:1
Let us not be weary in doing good, for we will reap in due season, if we don't give up. — Galatians 6:9

Author
Charles Spurgeon
Summary: We should not expect an immediate reward for the good we do or limit our efforts to those who seem to respond favorably. We are to do good even to the ungrateful and teach the indifferent and the obstinate: work that seems wasted may bear fruit in due time. Our task is to sow today; it is for God to fulfill his promise. Though months or years may pass before we see the result, his word will not be in vain. So we practice patience and devote ourselves to obeying the command today.
We should not expect to see an immediate reward for all the good we do; nor should we confine our efforts to places and people that seem likely to reward our labors. The Egyptian casts his seed upon the waters of the Nile, an act that would seem to be a sheer waste of grain. But in due time the river’s floodwaters recede, and the rice and other grains sink into the fertile mud, and soon a harvest is produced. Today we must do good to the ungrateful and the ungodly. We must teach the indifferent and the obstinate. Unpromising waters may cover promising ground. Nowhere will our labor in the Lord be in vain.
Our work is to cast our bread upon the waters; it is God’s part to fulfill the promise: “You will find it.” He will not allow His promise to fail. His good word that we have spoken will live, will be found, and will be found by us. Perhaps it will not happen yet, but one day we will reap what we have sown. We must exercise our patience; for perhaps the Lord will exercise it. “After many days,” says Scripture, and in many cases those days turn into months and years, and yet the word remains true. God’s promise will be fulfilled; we must concern ourselves with keeping the precept, and keeping it today.
Source: The Chequebook of the Bank of Faith. Translated by Allan Roman.
http://www.spurgeon.com.mx/chequera/meditames/junio.pdf