Rashi on Numbers 25:1/רש"י על במדבר כ״ה:א׳

1בשטים — Thus was its name: SHITTIM.

2לזנות אל בנות מואב TO COMMIT WHOREDOM WITH THE DAUGHTERS OF MOAB — by the advice of Balaam, — as is related in the chapter חלק (Sanhedrin 106a).