Marah

bitterness, a fountain at the sixth station of the Israelites (Ex 15:23; 15:24; Num 33:8) whose waters were so bitter that they could not drink them. On this account they murmured against Moses, who, under divine direction, cast into the fountain "a certain tree" which took away its bitterness, so that the people drank of it. This was probably the 'Ain Hawarah, where there are still several springs of water that are very "bitter," distant some 47 miles from 'Ayun Mousa.

EBD - Easton's Bible Dictionary