Travels of Ali Bey in Morocco, Tripoli, Cyprus, Egypt, Arabia, Syria, and Turkey. Between the years 1803 and 1807.
[Ali Bey al-Abbasi was the pseudonym of Domingo Badía y Leblich (Barcelona; Spain, 1766 – Syria 1818)]
Author: Ali Bey, 1766-1818
Publisher: Philadelphia: Printed for John Conrad, at the Shakespeare buildings, James Maxwell, Printer
Year: 1816 (English translation)

Volume 2. pp. 379-381:
    I saw the Sultan Mustapha go to public prayer on a Friday in a mosque called Sultan Djeami, or the mosque of the Sultan, situated opposite one of the gates of the seraglio. The street which the Sultan had to traverse in his way thither, was lined with two rows of janissaries from the gate of the seraglio to that of the mosque. My interpreter and my janissary would not approach, because every body trembles at the name of the Sultan. As for myself I crossed the ranks, penetrated into the court of the mosque, and placed myself in the most advantageous situation to see his highness.
    Several of the great personages of the court first arrived in succession, mounted upon superb horses, richly caparisoned, and surrounded with servants on foot. They alighted at the door of the mosque, and the servants ranged their horses on one side in the court.
    The janissaries of the guard wear, as do all the Turks, a long robe, but of different colours, each according to his taste, without any other sign of distinction, than an extravagant cap of greyish white felt, the hinder part of which hangs behind and covers the back; there is a plate of metal before, which falls upon the forehead, and encloses, as if in a case, a wooden spoon of a rude shape, which each janissary is obliged always to carry with him. They march without arms, having only a little wand in their hands.
    Afterwards came ten superb parade horses, belonging to the Sultan, all very large and of different colours, with great horse-clothes richly embroidered in gold and silver, the saddles also were covered with a magnificent cloth.
    The Sultan mounted upon a superb horse, arrived in his turn, preceded by thirty Bostandjis guards, armed with small gilt halberts. On each side of the Sultan were four officers, who may be called the skreens of the Sultan, on account of the immense plumage they wore upon their heads, which so hides the person of his highness, that it is difficult to obtain a sight of him. I had however a good front view of him, and observed him attentively with my glass, as long as I could. His face is extremely long, and his nose also, though not very prominent; he has large eyes; the paleness of his complexion is relieved by a slight colour in his cheeks; he appeared to me to be of a middling height, not lusty, and rather lively. He was dressed in a simple pelisse, but his turban was ornamented with a superb rose of extremely large diamonds, and of a dazzling brilliancy. Upon his entrance into the court of the temple, he made a slight salute by placing his right hand upon his breast, and looking to the right and left. The most profound silence reigned until he had arrived at the door of the mosque; as soon as he alighted a dozen men who were at the door cried out "Long live the Sultan."
    After the Sultan came the chief of the black eunuchs, whose figure is really frightful. He was richly dressed, mounted upon a magnificent horse, similar to that of the Sultan, and surrounded by his servants on foot. In passing, he made continual bows to the right and left, with so exact a precision, that he might have been taken for an automaton.
Source



The procession described is pictured in 'The Sultan riding out in procession to Friday Prayers' in Stratford Canning's Pictures of Turkey c.1809
See also An extract on Janissaries and other soldiers from Constantinople Ancient and Modern, with Excursions to the Shores and Islands of the Archipelago and to the Troad by James Dallaway, 1797
Illustrations of Ottoman Janissaries (Janizary, Yeniceri)
Illustrations of Ottoman Costume & Soldiers