Wikipedia
The Mubarizun (, "Duelists") formed a special unit of the Rashidun army. It was composed of the elite warriors, who were champion swordsmen, lancers and archers. In pre-Islamic Arab, Byzantine and Sassanian warfare, battles usually began with duels between the champion warriors of the opposing armies. The Mubarizun (literally: duelers, also translated as "champions") were a recognized part of the Muslim army, whose purpose was to slay the champion warriors of the opposing army to undermine its soldiers' morale.
The Muslim army would typically begin battle with its soldiers first equipping their armor, assembling their units to their positions and lastly dispatching the Mubarizun. Mubarizun fighters were instructed to refrain from pursuing any defeated enemy champions more than two-thirds of the way to the enemy lines to avoid the risk of being cut off. After the conclusion of the dueling phase, the army would launch its general advance.