Sahih Bukhari

I asked Az-Zuhri, "Did the Prophet (ﷺ) ever offer the Fear Prayer?" Az-Zuhri said, "I was told by Salim that `Abdullah bin `Umar I had said, 'I took part in a holy battle with Allah's Messenger (ﷺ) I in Najd. We faced the enemy and arranged ourselves in rows. Then Allah's Messenger (ﷺ) stood up to lead the prayer and one party stood to pray with him while the other faced the enemy. Allah's Messenger (ﷺ) and the former party bowed and performed two prostrations. Then that party left and took the place of those who had not prayed. Allah's Messenger (ﷺ) prayed one rak`a (with the latter) and performed two prostrations and finished his prayer with Taslim. Then everyone of them bowed once and performed two prostrations individually.' "

Ibn `Umar said something similar to Mujahid's saying: Whenever (Muslims and non-Muslims) stand face to face in battle, the Muslims can pray while standing. Ibn `Umar added, "The Prophet (ﷺ) said, 'If the number of the enemy is greater than the Muslims, they can pray while standing or riding (individually).' "

Once the Prophet (ﷺ) led the fear prayer and the people stood behind him. He said Takbir (Allahu-Akbar) and the people said the same. He bowed and some of them bowed. Then he prostrated and they also prostrated. Then he stood for the second rak`a and those who had prayed the first rak`a left and guarded their brothers. The second party joined him and performed bowing and prostration with him. All the people were in prayer but they were guarding one another during the prayer.

On the day of the Khandaq `Umar came, cursing the disbelievers of Quraish and said, "O Allah's Apostle! I have not offered the `Asr prayer and the sun has set." The Prophet (ﷺ) replied, "By Allah! I too, have not offered the prayer yet. "The Prophet (ﷺ) then went to Buthan, performed ablution and performed the `Asr prayer after the sun had set and then offered the Maghrib prayer after it."