Musnad Abu Ya'la al-Mawsili 3:178

'Amr ibn Muhammad al-Naqid narrated to us, Muhammad ibn Fudayl ibn Ghazwan narrated to us, 'Abd al-Rahman ibn Ishaq narrated to us, from al-Nu'man ibn Sa'd, from 'Ali ibn Abi Talib, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Indeed, in Paradise there are chambers whose exteriors can be seen from their interiors, and whose interiors can be seen from their exteriors." A Bedouin said: "For whom are they, O Messenger of Allah?" He said: "For those who fed the food, spread the greeting of peace, and prayed at night while the people were sleeping."