Exodus 4:8-9

8God said, ‘The leaders may not believe you. They may not think that the miracle with the stick shows anything. But they may believe you when you do the second miracle.

9But they may still not listen to your message. If they will not even believe those two miracles, do this: Take some water from the River Nile and pour it on the dry ground. The water that you take from the river will become blood on the ground.’