Rashi on Genesis
באשר ה' אתו — means BECAUSE THE LORD WAS WITH HIM (i.e. באשר means because).
Siftei Chakhamim
Because God was with him. [Rashi knows this] because the ב of באשר ה' אתו cannot follow its usual meaning, [“when Hashem was with him”]. For it says afterwards, “Whatever he did, Hashem made him succeed” — not sometimes yes and sometimes no.