From Sahih Muslim (Hadith 6278):
A'isha reported:Never did I feel jealous of the wives of Allah's Apostle (ﷺ) but in case of Khadija, although I did no, (have the privilege to) see her. She further added that whenever Allah's Messenger (ﷺ) slaughtered a sheep, he said: Send it to the companions of Khadija I annoyed him one day and said: (It is) Khadija only who always prevails upon your mind. Thereupon Allah's Messenger (ﷺ) said: Her love had been nurtured in my heart by Allah Himself
Reference: Muslim 6278