At the Gül (Rose) Mosque. It is (according to Strolling through Istanbul) doubtfully identified as the Church of St. Theodosia and deserved a closer inspection, but I found it closed.
A viewer let me know:
Well informed Turkish sources mention the Gül Camii as a former Byzantine church (the ‘Aya Theodosia Kilisesi’). After the fall of Constantinopel, it was first turned into a depot of naval equipment; under Selim II (1566-1674) it became a mosque, that was restored in 1951.

On the picture: An Ottoman ‘çeşme’, build in the mosque’s wall.

A çeşme (a kind of fountain) is a piece of architecture which pours water into a basin to supply drinking water.
They were connected to springs or aqueducts. Until the late 19th century most ‘çeşme’ operated by gravity, and needed a source of water higher than the fountain, such as a reservoir or aqueduct, to make the water flow.
In addition to providing drinking water, fountains were used for decoration and to celebrate their builders, which were generally high Ottoman state officials, whose name would be mentioned in the foundation inscription on the ‘kitâbe’ (= cornerstone) - (here: in the wall above the fountain).

Correspondent: J.M.Criel, Antwerpen.