In the Prophet’s army...
In the Prophet’s army, there used to be two standards; Sa’d was the bearer of the Ansar’s standard, while Imam Ali (a.s.) was the bearer of the Muhajirin’s standard. Sa’d was a loyal companion of the Prophet (S). He was a well-educated person and often officiated as the Prophet’s scribe in writing down the Qur’anic revelations, writing letters, drafting deeds…etc. His tribe was well known for its generosity and hospitality.
Sa’d was among the people of Medina who were, by reason of their learning of ancient scriptures, aware that a Prophet would soon appear in the Arabia. When they heard about the Prophet (a.s.) and the religion he preached and the torture which he and his followers were subjected to, Sa’d along with some other learned persons from Medina came to the Prophet (S) and invited him to migrate to Medina.
In every skirmish, battle, or other confrontation with the opponents of Islam, Sa’d participated and fought valiantly. When Umar, Abu Bakr, and Abu Ubaidah ibn al-Jarrah went to the Saqifa to stake the claim for the Caliphate, Sa’d bin Ubadah challenged them saying that among the tribe of Quraish, if the people of Quraish were to have precedence, Imam Ali (a.s.) was the nominated by the Prophet (S) as his successor and that he was the most fit person to the post.
On hearing this, Umar was so infuriated that he shouted, “Kill Sa’d.” However, Sa’d’s tribesmen rescued him. Sa’d never recognized Abu Bakr or Umar as the Caliphs. He never mingled with them nor did he offer prayers behind them at any time. He was steadfast in his refusal to acknowledge Abu Bakr and later Umar as the Caliph. Umar asked Khalid bin al-Walid who secretly killed Sa’d bin Ubadah.
Though Sa’d was not the first in point of time to be martyred because of his love for Imam Ali (a.s.), he was the first one to openly oppose at the Saqifa the men who claimed the Caliphate. Immediately after the death of the Prophet (S), the affair at the Saqifa created a rift among Muslims. The Ansar had heard the Prophet (S) nominating Imam Ali (a.s.) as his successor.
They were aware of the virtue, caliber, and wisdom of Imam Ali (a.s.) and hence were ready to accept him as the successor to the Prophet (S). However, during the last days of the Prophet (S), the Ansar found that every order of the Prophet (S) was disobeyed and the parchment and pen he requested to write down his will was denied. Umar was at the head of this group (who opposed and disobeyed the Prophet).