[1] At Medina, Hudhayfah became a trusted and great companion of Muhammad, participating in all the military engagements except Badr.
Before the battle, Muhammad left al-Yaman, Hudhayfah's father, and Thabit ibn Waqsh with the other non-combatants because they were both quite old.
As the fighting intensified, al-Yaman and his friend decided that despite their age they did not want to miss the opportunity to participate.
Hudhayfah had three qualities which particularly impressed Muhammad: his unique intelligence, quick-wittedness, and his ability to keep a secret even under persistent questioning.
A notable policy of Muhammad was to utilize the special strengths of each companion, carefully choosing the right man for the right task.
Because of Hudhayfah's ability to keep a secret, Muhammad told him the names of the twelve hypocrites, a trust not bestowed upon any other companion.
The hypocrites, because of their secrecy and intimate knowledge of the developments and plans of the Muslims, presented a greater threat to the community than external enemies.
From this time onward, Hudhayfah was called The Keeper of the Secret of the Messenger of Allah, remaining faithful to his pledge of secrecy.
Umar, during his caliphate was only able to find out indirectly who the hypocrites were by monitoring Hudhayfah’s attendance at the funeral prayer following the death of a Muslim.
" Sahih al-Bukhari, 5:59:449Hudhayfah's special qualities were made use of by Muhammad at various times, including the battle of the Ditch.
A violent wind overturned their tents, extinguished their fires and pelted their faces with sand and dust.
At this stage of the confrontation Muhammad sent Hudhayfah into the midst of the enemy's positions to bring him information on their situation and morale before deciding his next move.
The hypocrites began to ask Muhammad for permission to leave, saying, 'Our houses are exposed to the enemy.'
Anyone who asked Muhammad's permission to leave was allowed and many sneaked away until we were left with about three hundred men.
Shortly afterwards, Abu Sufyan got up and addressed his men: 'O people of the Quraysh, I am about to make a statement to you which I fear would reach Muhammad.
Hudhayfah's experience with hypocrisy and his efforts to combat it gave a touch of sharpness and severity to his tongue.
He himself realized this and admitted it with a noble courage: "I went to [Muhammad] and said: 'O Messenger of God, I have a tongue which is sharp and cutting against my family and I fear that this would lead me to hell.'
Hudhayfah was one of the foremost military commanders in the expansion into Iraq, distinguishing himself at Hamadan, ar-Rayy, ad-Daynawar, and at the famous battle of Nihawand in December 641.
For the encounter at Nihawand against the Persian forces, Hudhayfah was placed second in command by Umar over the Muslims who numbered some thirty thousand.
Hudhayfah also took part in the multi-pronged advance into Armenia, leading a column into the Al-Lan mountains in 644.
When the news of his appointment spread, Ctesiphon crowds went to meet this famous companion of whose great role in the conquests of Persia was already a legend.