Imam Malik's Muwatta, Chapter: Jihad

Yahya related to me from Malik from Abu'n-Nadr, the mawla of Umar ibn Ubaydullah that he had heard that the Messenger of Allah, may Allah bless him and grant him peace, said over the martyrs of Uhud, "I testify for them." Abu Bakr as-Siddiq said, "Messenger of Allah! Are we not their brothers? We entered Islam as they entered Islam and we did jihad as they did jihad." The Messenger of Allah, may Allah bless him and grant him peace, said, "Yes, but I do not know what you will do after me." Abu Bakr wept profusely and said, "Are we really going to out-live you!"