“The world is three days: yesterday, which has gone with all it held; tomorrow, which you may never reach; and today — so work in it.”

Ibn Abi al-Dunya (d. 281 AH / 894 CE) records this with its chain back to al-Hasan al-Basri in his al-Zuhd, and the very next report gives it again from a different student in almost the same shape: “yesterday has passed with what was in it, and tomorrow you may not reach — so look to what you will do today.” Two transmissions agreeing like that is a good sign the wording is his.