Musnad Ahmad 658:133

Abu Abd al-Rahman al-Muqri and Husayn ibn Muhammad—the meaning being the same—said: Abu Raja’ Abdullah ibn Waqid al-Harawi narrated to us, saying: Muhammad ibn Malik narrated to us, from al-Bara’ ibn ‘Azib, who said: While we were with the Messenger of Allah (may Allah bless him and grant him peace), he caught sight of a group and said: "For what have these gathered?" It was said: "For a grave they are digging." He said: The Messenger of Allah (may Allah bless him and grant him peace) became alarmed, and he hurried ahead of his companions until he reached the grave, and he knelt by it. He said: I stood facing him from in front of him to see what he was doing. He wept until he moistened the earth with his tears. Then he turned to us and said: "O my brothers, for the likes of this day, prepare!"