The Holy One, the blessed One said to Israel, "Even if you find the land full of all good things, do not say, 'We will sit and not plant'; rather be diligent in planting... Just as you came and found trees planted by others, you must plant for your children." A person must not say, "I am old, how many years will I live? Why should I stand and exert myself for others? Tomorrow I will die." You must not excuse yourself from planting. As you found trees, plant more, even if you are old.
Midrash Tanhuma, Kedoshim: 8