Musnad Ahmad 27:1195

Rabi' ibn Ibrahim narrated to us, Abd al-Rahman ibn Ishaq narrated to us, from Abdullah ibn Dinar, from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not enter upon the people who are being punished, unless you are weeping, lest what afflicted them should afflict you."