Musnad Ahmad 27:992

'Affan narrated to us, 'Abd al-'Aziz ibn Muslim narrated to us, 'Abdullah ibn Dinar narrated to us, from 'Abdullah ibn 'Umar, who said: The Messenger of Allah (may Allah bless him and grant him peace) said, while he was at al-Hijr: "Do not enter upon these people who were punished, unless you are weeping. If you are not weeping, then do not enter upon them, lest there befall you the like of what befell them."