The ablutions fountain at the north-eastern side of the Rüstem Paşa.

A Şadırvan (from the Persian ‘Sadirvan’) is a type of fountain that is usually built in the yard or entrance in front of mosques or other buildings where ritual prayers can be held, with the main purpose of providing water for drinking or ritual ablutions to several people at the same time, but also as decorative visual or sound element. It is a typical element of Ottoman architecture. It is also called ‘abdest alma çeşmesi’ (= fountain for ablutions).

Regarding Ritual ablutions:
‘Wudu’ (Arabic: الوضوء‎ al-wuḍūʼ) or in turkish: ‘abdest’?is the Islamic procedure for washing parts of the body, typically in preparation for formal prayers. The turkish word is from Persian origin (âb = water & dest = to take) and was introduced in Seljuk times (11th – 13th century).

For Sunni moslims, the procedure (which can be characterised as ‘small washings’) consists of four actions:
Washing the face once; washing both arms including the elbows once; performing ‘masah’ of one-fourth of the head; washing both the feet once up to and including the ankles.
‘Masah’ of the head: wet hands should be passed all over the head, with a deliberate stroke downwards from the top of the head; then index fingers are placed in ear canal while thumbs pass behind ears and lobes; then swipe back of hands over neck nape. This is done in one continuous motion, without refreshing the hands with water for each component.

Correspondent: J.M.Criel, Antwerpen.
Source: (among others) Wikipedia.