Exodus 15:23

And they will come to Marah, and they will not be able to drink the waters of Marah, for they are bitter: for this he called the name bitterness.

Numbers 33:8

And they will remove from before Hiroth, and they will pass through the midst of the sea of the desert, and they will go a way of three days in the desert of Etham, and they will encamp in Marah.

Ruth 1:20

And she will say to them, Ye shall not call to me Naomi; call to me, Mara: for the Almighty made bitter to me greatly.

Treasury of Scripture Knowledge

Summary

And when they came to Marah, they could not drink of the waters of Marah, for they were bitter: therefore the name of it was called Marah.

Bible References

Marah

Numbers 33:8
And they will remove from before Hiroth, and they will pass through the midst of the sea of the desert, and they will go a way of three days in the desert of Etham, and they will encamp in Marah.
Ruth 1:20
And she will say to them, Ye shall not call to me Naomi; call to me, Mara: for the Almighty made bitter to me greatly.