Williams, 28, was not injured in the robbery and is doing well, the source said.
TMZ Sports first reported the incident and details of the robbery.
Williams led the Chiefs in rushing last season with 498 yards and five touchdowns. He also scored twice on receiving touchdowns.
Williams was one of the Chiefs' stars in their Super Bowl LIV victory over the San Francisco 49ers. He rushed for 104 yards and in the fourth quarter scored their final two touchdowns, one on a 5-yard reception to put the Chiefs in the lead and one on a 38-yard run to clinch the victory.