This is one of the 13 mosaics along the upper band of the left wall of the nave that follow the life of Christ. He usually blesses a person as here, with the fingers of his right hand held in the "Old Believers" configuration.
The episode is narrated in John 4:4-29. Jesus promises "living
water" to the Samaritan woman, who then goes into the village with
the news of this man. "Could he be the Messiah?"
See also a view of the entire
left wall with commentary on the iconography of Jesus and
the apostles in these mosaics.
This image in full resolution
More of the Miracles of Jesus