Musnad Ahmad 1060:1

'Ali ibn 'Asim narrated to us, 'Abdullah ibn 'Uthman ibn Khuthaym narrated to us, from 'Uthman ibn Jubayr, from Abu Ayyub al-Ansari, who said: A man came to the Prophet, may Allah bless him and grant him peace, and said: "Advise me and be concise." He said: "When you stand in your prayer, pray the prayer of one who is bidding farewell. Do not speak with speech for which you will have to apologize tomorrow, and gather despair regarding what is in the hands of the people."