This title has its biblical origin {in Gen. 32:24-31}, where Jacob after wrestling with the angel and receiving the name Israel, exclaims "I have seen God"... The etymology may derive either from reading Yisrael as a contraction of `is raah `el "a man who saw God" or the equivalent of yasur `el, "he sees God".
(Philo of Alexandria: The Contemplative Life, the Giants, and Selections, pp. 351-352, footnote 240)