And when they came to Marah, they could not drink the waters of Marah, for they were bitter; therefore it was named Marah. So the people grumbled at Moses, saying, “What shall we drink?” Then he cried out to the LORD, and the LORD showed him a tree; and he threw it into the waters, and the waters became sweet. (Exodus 15:23-25)
Moses had just led the Israelites out of Egypt and across the Red Sea that God parted for them. All the women had taken up timbrels and followed Miriam dancing and celebrating, exalting God for rescuing them from the Army of the Pharaoh. From there, Moses began to lead the newly freed nation into the wilderness.
They had wandered only three days, but it was three days without water. They found water at Marah, but it was too bitter to drink. So, the people began to get restless and “grumbled” to Moses. This is only the beginning of the grumbling, a theme that would continue throughout the years wandering in the wilderness. Even after God did miraculous things, like part the Red Sea and rescue them from certain capture and calamity, the people were quick to fall back to the habit of complaining.