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.
Correspondent: J.M.Criel, Antwerpen.
Source: (among others) ‘Türkiye Tarihi Yerler Kılavuzu’ – M.Orhan Bayrak, Inkılâp Kitabevi, Istanbul, 1994.