Rashi on Genesis 13:7/רש"י על בראשית י״ג:ז׳

1ויהי ריב AND THERE WAS A QUARREL because Lot’s shepherds were wicked men and grazed their cattle in other people’s fields. Abram's shepherds rebuked them for this act of robbery, but they replied, “The land has been given to Abram, and since he has no son as heir, Lot will be his heir: consequently this is not robbery”. Scripture, however, states: “The Canaanite and the Perizzite abode then in the land”, so that Abram was not yet entitled to possession (Genesis Rabbah 41:5).