The mosque (sometimes called ‘Bekçi Camii’/Mosque of the Caretaker) dates from the reign of sultan Çelebi Mehmet I (1413-1421). It was built, in the name of Ibni Bezzaz, by Hafizüddin Efendi. Ibni Bezzaz(i) Kerderi was a major scholar of the Hanefi school of law, who was born in Khwarezm (Central Asia) and came to Anatolia; he died in Bursa in 1424.
The square prayer hall (7,89 x 7,94 m) is covered by a dome, and is preceded by a porch/vestibule (7,89 x 3,40 m) on its north side. The building was severely damaged by the 1855 Bursa earthquake; its present state is the result of a thorough restoration in 1979.

On the picture: The inscription above the entrance, written in sülüs script. It says: “Ínnessalâte kânet alel mü’minine kitaben mevkûten”; which means: ‘For all believers, true prayer at the right time was made obligatory and (so it was) written (down)”.

Sülüs (from Arabic: ثلث‎ ṯuluṯ "one-third"), also referred to as ‘thuluth’, is a script variety of Islamic calligraphy invented in Persia, which made its first appearance in the 11th century AD. The straight angular forms of the older Kufic script were replaced in the new script by curved and oblique lines. In Sülüs, one-third of each letter slopes, from which the name (meaning "a third" in Arabic) comes. It is a large and elegant, cursive script, used in medieval times on mosque decorations and inscriptions.

Correspondent: J.M.Criel, Antwerpen.
Sources: ‘Vakıf Abideler ve eski Eserler (volume III)’ - Vakıflar Genel Müdürlüğü, Ankara 1983 , Wikipedia
& Website of ‘liveinbursa.com’.