Dividing the Islamic Army into Phalanxes and Departure to the Battle-field
Ascending the pulpit at the Friday congregational prayer, the Prophet [pbuh] urged the people in his sermon to fight courageously. "If you remain steadfast," he said "you will be helped by the Power of the All- Mighty." Then he commanded his men to make ready for the battle. Most of them rejoiced greatly.
He led the afternoon prayer with crowds of people. Then he entered his house accompanied by his two friends Abu Bakr and ‘Umar. They helped him dress and wear his headcloth. He armed himself and wore two armours one over the other. He wore his sword and went out to meet people.
People were waiting for him impatiently. Sa‘d bin Mu‘adh and Usaid bin Hudair blamed people for pressing on the Prophet [pbuh]. They said: "You have forced the Messenger of Allâh [pbuh] to fight the enemy outside Madinah." Therefore they were determined to leave the whole matter to the Prophet [pbuh], and blamed themselves for what they had already done. When the Prophet [pbuh] came out, they said: "O Messenger of Allâh, we should have not disagreed with you. So, you are free to do what you desire. If you prefer to stay inside Madinah we will stay with you. Upon this the Messenger of Allâh [pbuh] remarked: "It does not become a Prophet that once he had put on armour, he should take it off, until Allâh has decided between him and the enemy." [Quoted by Ahmad, Nasa'i, Hakim and Ibn Ishaq]
The Prophet [pbuh] divided his army into three battalions:
The army consisted of a thousand fighters; a hundred of them armoured; another fifty horsemen. [Al-Huda 2,92] He appointed Ibn Umm Maktum to lead the people in prayer in Madinah. Departure was announced and the army moved northwards with the two Sa‘ds, who were armoured, running in front of the army.
Upon passing along Al-Wada‘ mountain trail he saw a well-armed battalion, which were detached from the main body of the army. The Prophet [pbuh] inquired who they were and he was told that they were Jews and were allies of Al-Khazraj. They told him that they wanted to contribute to the fight against the idolaters. "Have they embraced Islam?" The Prophet [pbuh] asked. "No," they said. So he refused admitting them and said that he would not seek the assistance of disbelievers against the idolaters.