Talmud Ta'anit 23a

While walking along a road, a sage saw a man planting a carob tree. He asked him: "How long will it take for this tree to bear fruit?" "Seventy years," replied the man. The sage then asked: "Are you so healthy a man that you expect to live that length of time and eat its fruit?" The man answered: "I found a fruitful world, because my ancestors planted it for me. Likewise I am planting for my children."