After the conquest of Isfahan, when the north of Persia had been cut off from the south, Umar ordered a march against Fars the southern province of Persia.
The operation against Fars was to be undertaken in a series of campaigns. In the first campaign a corps under Mujashe bin Masud advanced in the district of Ardsheer Khurra. There was a confrontation at Tawwaj where the Muslims defeated the Persian force.
From Tawwaj the Muslim army proceeded to the town of Sabur. The town was besieged. Brought to bay the Persians laid down their arms and submitted to Muslim rule.
In the second campaign led by Othman bin Abul Aas, and starting from where Mujashe bin Masud had left off the Muslim army advanced to Jor. It was a city to the south of Shiraz and some distance away from the Persian Gulf. The Persian garrison at Jor offered resistance but they were soon overwhelmed and the city submitted to the Muslim rule.
From Jor the Muslim army struck north and occupied the city of Shiraz. >From Shiraz the Muslim army struck north east and occupied Persepolis the ancient capital of Persia.
The third campaign led by Sariyah bin Zuneim launched from Persepolis was directed against Fasa and Darab. It was a hilly region. At a place near Fasa the Muslim army was stranded and exposed to great danger. That day was Friday and at that time Umar was delivering the Friday sermon in the Prophet's mosque at Madina. Suddenly in the course of the sermon, Umar shouted 'Sariyah to the hill, to the hill'. When the prayer was over the people gathered round Umar and enquired what was the significance of his word 'Sariyah to the hills' in the course of the sermon.
Umar said that while he was delivering the sermon he saw that the Muslim army under Sariyah fighting on the Fasa front, was stranded. He advised them to seek the protection of the hills.
It is related that on the battledfield Sariyah heard the voice of Umar and he took to the hill as desired. That turned the tide of the battle, and the Muslims won the day. When after many days the messenger from Sariyah reached Madina to convey the news of the victory of Fasa, he declared on oath that on the day of the battle at the time of Friday prayers they had actually heard the call of Umar enjoining them to take to the protection of the hill, and that they had acted accordingly.
After Fasa, the Muslims marched to Darab and there too the Persians surrendered after offering feeble resistance.
The fourth campaign in this series was carried by Suhail bin Adi. A column under Suhail marched to Kirman. The Persians offered resistance but when their Commander was killed on the battlefield, they lost heart and submitted to the Muslims. With Kirman as the base, the Muslim force marched against the towns of Jiraft and Sirjan without any difficulty. The entire province of Fars now lay at the feet of the Muslims.
After the conquest of Fars the Muslims turned to the province of Sistan, the home of the legendary hero Rustam of Shahnama fame. The attack on Sistan was led by Asim bin Amr. At the border of Sistan the army of Asim were obstructed by a Persian force. In a violent attack launched by the Muslims the Persian forces were scattered, and the Muslim forces advanced to Zaranj. It was a fortified town, and the Persian garrison shut itself in the fort. The town was besieged by the Muslims and then carried by assault. The Persians surrendered, and the entire province of Sistan came under Muslim occupation.