Ex 15:22-24

Moses led Israel from the Red Sea on to the Wilderness of Shur. They traveled for three days through the wilderness without finding any water. 23 They got to Marah, but they couldn't drink the water at Marah; it was bitter. That's why they called the place Marah (Bitter). 24 And the people complained to Moses,

After God had done incredible things for them they complaied…