The inscription here is Qur'an II, sura al-baqara, v. 255 (ayat al-kursi).
This marble panel is attributable to Mehmed Tahir Efendi (d.1846), the most prominent student of Mahmud Celâleddin Efendi (d.1829). Mahmud Celâleddin developed a distinctive style, referred to by Derman as a 'hard and static' and relating more closely to the calligraphic mode of Ahmad Karahisari or Yaqut al-Musta'simi than to the cursive naskh of Sheikh Hamdullah that was favoured by his contemporaries. Some say that this diversion from the norm was reflective of Celâleddin's stubborn and obstinate character (M. Ugur Derman, Letters in Gold, exhibition catalogue, New York, 1998, p. 108).
Mehmed Tahir continued Celâleddin's distinctive style throughout his life. Such was his preservation of Mahmud Celâleddin's style that it is in fact not easy to distinguish between the works of Mehmed Tahir Efendi and those of his celebrated master. Mehmed Tahir's panels in jali thuluth are most common and an example of his work is found in the inscriptions of the Defterdar mosque built in 1826 near the Harem wharf in the district of Üsküdar. Mehmet Tahir was also the calligraphy master of Sultan Abdulmecid (r. 1839-1861), and it was through Abdulmecid's appreciation of Celâleddin and Mehmed Tahir's distinctive style that it was given a brief lease of life before it became obsolete.
The calligraphy of the present panels very much follows this distinctive school. The bold upstrokes and sharp finish of the letters support the attribution. In the style derived from Yaqut naskh was grouped into single words, as in the present panel, whereas in the style derived from Sheikh Hamdullah and perpetrated by later calligraphers, letters and words develop a fluidity and organic unity not seen here (Oktay Aslanapa, Turkish Art and Architecture, London, 1971, p. 324). This panel appears to be a survival from an idiosyncratic and short lived calligraphic style, particularly associated with the reign of Sultan Abdulmecid.