On the authority of al-Baraa ibn ‘Aazib (RadiAllahu anhu) who said: While we were with the Messenger of Allaah (Sallellahu alaihi wa sallam), he suddenly looked towards a group (of people) and said:
“”For what reason have they gathered here?””
It was said: In order to dig a grave.
So the Messenger of Allah (s.a.w.s.) became alarmed and startled and he quickly went ahead of his Companions until he reached the grave, then he knelt upon it, and I turned my face towards him ,in order to see what he was doing. He cried until the earth became wet with his tears, then he turned to us and said:
“”O MY BROTHERS! PREPARE FOR A DAY LIKE THIS.””
[Al-Bukhari in at-Taareekh, Ibn Maajah, Ahmad and others. It is a hasan hadith as indicated by Shaykh al-Albaani in Silsilah al-Ahaadith as-Sahihah]