The stream's source was inside the temple.
from under the south side of the temple, south of the altar: In Solomon's Temple, this spot held the Sea, a large bronze pool for cleansing water (1 Kings 7:23, 39). The Sea also had symbolic meaning. In the ancient world, the sea often represented chaos (disorder and danger). By placing the Sea in the temple, it showed that God had power over chaos within his ordered world.
In Ezekiel’s vision, the Sea is no longer a still pool. Instead, it becomes a flowing river that gives life. This image recalls the river in Eden (see Genesis 2:10–14; Psalm 46).