Sahih Muslim
The Book Pertaining to Piety and Softening of Hearts (Kitab Al-Zuhd wa Al-Raqaiq)
Book Number: 042 / Hadith Number: 7089
'A'isha reported: We the family of Muhammad (may peace be upon him) used to spend (the whole) month in which we (did not need to) kindle the fire as (we had nothing to cook) ; we had only dates and water (to fill our bellies).