Musannaf Ibn Abi Shayba 33:1366

Husayn ibn Ali narrated to us, from Za'idah, from Abd al-Malik ibn Umayr, who said: A man advised his son and said: "O my son, manifest despair regarding what is in the hands of the people, for it is wealth. And beware of seeking needs [from others], for it is present poverty. And beware of that for which one must offer an excuse in speech. And when you pray, pray the prayer of one who is bidding farewell, not thinking that you will return. And if you are able to be better today than you were yesterday, and better tomorrow than you are today, then do so."