Firaz. By the end of 633 C.E., the Muslims were the masters of the Euphrates valley. In this valley, Firaz at the outermost edge of the Persian Empire still had a Persian garrison. Khalid decided to drive away the Persians from this outpost as well. Khalid marched to Firaz with a Muslim force and arrived there in the first week of December 633 C.E. Firaz was the frontier between the empires of Persia and Byzantium, and the garrisons of the Persians as well as the Byzantines were cantoned there. In the face of the Muslims, the Byzantine garrison decided to come to the aid of the Persian garrison. The united forces of the Persians, the Byzantines, and the Christian Arab auxiliary, were ten times the number of the Muslim force. Impressed by the imposing array of the coalition, the Byzantine General sent a haughty message to Khalid, demanding an unconditional surrender. Khalid replied that he would give reply on the battlefield.

The battle of Firaz. Khalid gave the enemy the option to cross the Euphrates. As soon as the enemy had crossed the Euphrates, Khalid commanded the Muslim force to go into action. The united forces of the Persians and the Byzantines had the river at their back, and the position was similar to that at the battle of Mazar. At Firaz, Khalid adopted the same tactics as he had adopted at Mazar. As the front ranks of both the forces committed themselves in the fighting, Khalid fixed his enemy on either flank with the help of his rear wings. Making a swift lighting movement, the Muslims dashed for the bridge on the river, and succeeded in occupying it. The enemy was thus held in a pincer movement. The Muslims intensified the attack and closed the noose round the neck of the enemy. In the mortal conflict that ensued, the enemy soon lost the ground. The withdrawing forces of the Persians and the Byzantines either jumped into the river in a state of horror or confusion or allowed themselves to be squeezed to death. It was a bloody battle, and over fifty thousand men of the enemy fell on the battleground. The battle was soon over and Firaz, the last stronghold of the Persians, fell to the Muslims. The battle of Firaz added further luster to the Muslim arms.

Khalid's pilgrimage to Makkah. In the beginning of the battle of Firaz when the odds appeared to be against the Muslims, Khalid undertook an oath that if he was victorious, he would undertake pilgrimage to Makkah, the House of God. After the victory of Firaz, Khalid stayed at Firaz for some days and made the necessary arrangements for the administration of the territory. In January 634 C.E., while a garrison was kept at Firaz, orders were issued to the main Muslim army to return to Al Hirah. Khalid stayed behind with the rear of the army. As the army moved forward on the road to Al Hirah, Khalid separated himself from the army, and took an unfrequented route to Makkah with a small escort. Khalid reached Makkah in time to perform the 'Hajj'. After performing the pilgrimage secretly and fulfilling his vow, Khalid and his party rode back to Al Hirah. Before the last contingent of the main army from Firaz had entered Hirah, Khalid was also there, as if he had been ail the time with the rear guard. Although Khalid had taken pains to ensure that he was not recognized at Makkah, news was nevertheless carried to Abu Bakr that leaving his charge in Iraq, Khalid had visited Makkah incognito. When Khalid reached Al Hirah, he got a letter from Abu Bakr asking him not to indulge in such adventure again.