In this image the Angel Gabriel is shown delivering the Word of God to Mohammed. In fact, God's revelations took place over several years. Here he is receiving the Eighth Surat, surrounded by those who had already started to follow him. The four men kneeling to the right were the four Caliphs who followed Mohammed. At the front, in green, is Ali, who married the Prophet's daughter Fatima.
Korans were never illustrated with figures, but secular manuscripts sometimes were - this artist however chose to veil Mohammed's face rather than to fully depict it. The story of the Life of the Prophet was included in many histories of the world, such as this 16th century Ottoman manuscript now in the Louvre (inv. MAO 708). The Ottomans tended to be iconophiles, so the lack of a face here is probably at the request of the manuscript's patron.