The Battle of Tabouk (also called the Battle of Tabuk) took place in October 630 AD. According to Islamic sources the followers of Muhammad were heavily outnumbered by the army of the Byzantine Empire and they withdrew to Medina before the battle began. Tabouk is in northwestern Saudi Arabia.
The battle of Tabouk took place in the 9th year of the Muslim calendar. According to the Ar-raheeq Al-makhtum, an Arabic historiographical work, Heraclius, then Emperor of the Byzantine empire, had decided that reducing the growing Muslim power had become an urgent necessity and the conquest of Arabia should, in his opinion, be achieved before the Muslims became too powerful to conquer and raise troubles and unrest in the adjacent Arab territories. According to the Muslim accounts, the Byzantine Emperor mustered a huge army of Byzantine soldiers and pro-Roman Ghassanid tribes to launch a decisive military attack against the Muslims.
A magnified rumour of the danger threatening Muslim life was carried to Makkah by some Nabateans who traded from Syria to Medina. They carried rumours of Heraclius` preparations and the existence of an enormous army said to number anywhere from forty thousand to several hundred thousand besides the Lukham, Judham and other tribes allied to the Byzantines.
Muhammad thought that if he tarried or dealt passively with the situation the Byzantines would be able to invade the Muslim-controlled provinces and even get as far as Madinah. This would create the most awful impression of Islam among newly conquered tribes as well as of the Muslims` military credibility.
When Muhammad had made up his mind and took his final decision, he ordered his Companions to get ready for war and sent for the Meccans and the other Arab tribes asking for their assistance.
Muhammad marched northwards to Tabouk. The army that numbered thirty thousand fighters was a great one, when compared with the previous armies of Islam. Muslims had never marched with such a great number before.
After arriving at Tabouk and camping there, the muslim army was ready to face the enemy. However there was no Byzantine Army to be seen. According to Muslim histories, upon learning of the Muslims` march the Byzantines and their allies were so terrified that none of them dared set foot out to fight. On the contrary they scattered inside their territory.
It is sometimes claimed that this "battle" brought, in itself, credit to the Muslim forces that had gained military reputation in the remote lands of the Arabian Peninsula.
The only gain that the Muslims made was after they threated locals Arab tribes. The head of the Ailah, Yahna bin Rawbah came to Muhammad and made peace with him and paid him the jizyah as tribute. Both the Jarba`and Adhruh peoples paid him tribute as well.
Muhammad gave each tribe a letter of guarantee, similar to Yahna`s, in which Arab histories claim he said: "In the Name of Allah, the Most Beneficent, the Most Merciful.
This is a guarantee of protection from Muhammad to Yahna bin Rawbah and the people of Allah; their ships, their caravans on the land and sea shall have custody of Muhammad , he and whosoever are with him of Ash-Sham people and those of the sea. Whosoever contravenes this treaty , his wealth shall not save him; it shal be the fair prize if him that takes it. Now it should not be lawful to hinder the men from any springs which they have been in the habit of frequenting, nr from any journeys they desire to make, whether by sea or by land."
The tribes, who used to ally the Byzantines, became certain that their dependence on their former allies had come to an end. Therefore they turned to the Muslims. The Islamic state had therefore enlarged its borders to an extent that it touched the Byzantine borders.
The Expedition of Tabuk from Restatement of History of Islam at Al-Islam.org