Rashi on Numbers 33:49/רש"י על במדבר ל״ג:מ״ט

1מבית הישמת עד אבל השטים [AND THEY ENCAMPED] … FROM BETH-JESIMOTH EVEN UNTO ADEL-SHITTIM — Here it tells you that the extent of Israel’s camp was twelve miles, for Rabba bar bar Chana said: “I myself have seen that place etc., and it is three Parsangs (12 miles) square” (Eruvin 55b).

2אבל השטים — The plain of Shittim had the name אבל (cf. Rashi on Genesis 14:6).