Prepare for a day like this...

On the authority of al-Baraa ibn 'Aazib (radhiAllaahu 'anhu) who said: While we were with the Messenger of Allaah (sallAllaahu 'alayhi 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 Allaah (sallAllaahu 'alayhi wa sallam) 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 (sallAllaahu 'alayhi wa sallam), in order to see what he was doing. He (sallAllaahu 'alayhi wa sallam) 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-Bukhaaree in at-Taareekh, Ibn Maajah, Ahmad and others. It is a hasan hadeeth as indicated by Shaykh al-Albaanee in Silsilah al-Ahaadeeth as-Saheehah