Sayyiduna al-Bara' bin Azib (may Allah be pleased with him) narrates: We were with the Messenger of Allah (peace and blessings be upon him) at a funeral. When we reached the grave, he (peace and blessings be upon him) scattered dust onto the grave. So I went around and came in front of him, and he (peace and blessings be upon him) wept (so much) that the soil became wet. Then he (peace and blessings be upon him) said: "O my brothers, the same matter is (to happen to us), so prepare for it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6515