SAAD IBN UBADA WAS THE LEADER OF THE KHAZRAJ TRIBE OF MEDINA. The Khazraj and the Aus, the two tribes of the Ansar, had distinguished themselves by their services to Islam. Their services were acknowledged even by Abu Bakr when he was bickering and dickering with them in Saqifa.
In the battles of Islam, the Ansar were invariably in the forefront. They fought against the combined might of all the idolaters of Arabia. Abu Qatada, an Ansari, claimed with truth that no tribe in all Arabia had produced more martyrs for Islam than the Ansar. More Ansaris were killed in defending Islam than men of any other tribe.
There was a time when Islam was "homeless." No tribe in Arabia offered sanctuary and hospitality to Islam and to its Prophet except the Ansar. They invited Muhammad to be their guest, and they made him the king of their city – Yathrib (Medina).
It was the city of the Ansar that won the honor and the glory of being the cradle and the capital of Islam. It was in their city that Muhammad, the Messenger of God, built the "edifice" of the first and the last Kingdom of Heaven on Earth.
In A.D. 623 (2 Hijri), Muhammad led an expedition to Waddan, and he appointed Saad ibn Ubada as governor of Medina during his own absence. Saad, therefore, was the first governor of Medina.
In the battle of Uhud, the Apostle of God gave the banner of the Khazraj to Saad. In the same battle, the Muslims were defeated. With the exception of 14 companions, everyone else fled from the battlefield. Saad was one of these 14 heroes who fought against the enemy, and defended the Apostle of God.
In the expedition of Mustaliq and in the siege of Medina (the battle of Khandaq), Saad carried the banner of the Ansar.
In 6 Hijri the Apostle went on a campaign and he appointed Saad governor of Medina in his absence.
The Ansar had two leaders, Saad ibn Ubada and Saad ibn Mua'dh. Saad ibn Mua'dh died from a wound he received in the battle of Khandaq. After his death, Saad ibn Ubada was the sole leader of the Ansar.
In Saqifa, the Ansar told Saad that he was the worthiest man to be the khalifa, and they declared their support for his candidacy.
Saad was famous for his generosity. Sometimes he entertained as many as 80 guests. Anyone – friend or stranger, could count on his hospitality.
Saad refused to take the oath of allegiance to Abu Bakr. Three years later, he left Medina, went to Syria and settled there. He was in Syria when he was hit by an arrow shot by some unknown person, and he thus died in mysterious circumstances.
Saad ibn Ubada was the first and the last Ansari ever to become a candidate for khilafat. He didn't become a khalifa. In Saqifa, the door of khilafat was slammed in the face of the Ansar, and they were shut out for all time.