Musnad Ahmad 1060:0

Abu Ayyub al-Ansari reported: A man came to the Prophet, may Allah bless him and grant him peace, and said, "O Messenger of Allah, teach me and be brief." The Prophet, may Allah bless him and grant him peace, said: "When you stand in your prayer, pray as if it is a farewell prayer. Do not speak a word for which you will have to apologize tomorrow, and despair of what is in the hands of people."