Musnad Ahmad 1153:301

Abu Usamah narrated to us, Hisham narrated to us, from his father, from Aisha, who said: "I did not feel jealous of any woman as I felt jealous of Khadija, and she had perished three years before he married me, because of what I used to hear him mention her. And his Lord, the Mighty and Majestic, had commanded him to give her glad tidings of a house of hollowed pearl in Paradise. And he would indeed slaughter a sheep, then send it to her friends."