See Barbara Gaehtgens, Adrian Van der Werff, pp. 272-273 where she identifies this plaque as inspired by the painting by Van der Werff (1659-1722) in the Gemäldegalerie, Dresden.
Ishmael was Abraham's first son, born by Hagar, the Egyptian handmaiden to Sarah, Abraham's wife. Ishmael mocked Abraham's second son Isaac (born by Sarah) and Sarah asked Abraham to banish them both. Provided with bread and water, Hagar and Ishmael were banished to the desert of Beersheba (Genesis, 21:9-21).