Sahih Muslim
The Book Pertaining to the Merits of the Companions (Allah Be Pleased With Them) of the Holy Prophet (May Peace Be Upon Him) (Kitab Al-Fadail Al-Sahabah)
Book Number: 031 / Hadith Number: 5974
'A'isha reported: Never did I feel jealous of any wife amongst the wives of Allah's Apostle (may peace be upon him) as I feel in case of Khadija (though I had never seen her), for he praised her very often.