When is it Permissible to Pray the Fear Prayer

Question :

Is it permissible for those who are working on weapons at the front lines to pray the Fear Prayer? And how can that be if there is no fighting taking place?

Answer:

In this case, they cannot pray the Fear Prayer; only when they are in lines facing the enemy or if they fear attack from them, they can do so. This is because Allâh Almighty says:
"When you (O Muhammad) are among them, and lead them in the Salah, let one party of them stand up (in prayer) with you, taking their arms with them; when they finish their prostrations, let them take their positions in the rear and let the other party come up which has not yet prayed, and let them pray with you, taking all the precautions and bearing arms. Those who disbelieve wish, if you were negligent of your arms and your baggage, to attack you in a single rush."

In the Two Sahihs there is a narration from Salih bin Khawwat about those who prayed the Fear Prayer with the Prophet ﷺ during the battle of Thatur-Riqa'. A group of the Companions lined up behind the Prophet meanwhile, another group of Companions faced the enemy. The Prophet ﷺ prayed one Rak'ah of prayer with the first group; he remained standing as they completed the second Rak'ah themselves, after which they departed and took their position to face the enemy. The second group came, he prayed the Rak'ah that remained, then remainedseated while they finished by themselves, then he said the Taslim with them. This is the wording of Muslim.

Also in the Two Sahihs, it is recorded that Ibn 'Umar said: "I participated in a battle with Allah's Messenger ﷺ in Najd. We faced the enemy and arranged ourselves in rows. Then Allah's Messenger stood to offer the Salah and one party stood to offer the Salah with him while the other faced the enemy. Allâh's Messenger and the former party bowed and performed two prostrations. Then that party left. Then they took the place of those who had not offered Salah. They (the second group) joined Allah's Messenger who prayed one Rak'ah and performed two prostrations and finished his Salah with the Taslim. Then everyone of them bowed once and performed two prostrations individually." This is the wording of Al-Bukhari.

It is also recorded from Jabir, that he said, "I participated with Allah's Messenger in the Fear Prayer. We formed two rows, a row was behind Allah's Messenger with the enemy between us and the Qiblah. The Prophet said the Takbir and we all said the Takbir. Then he bowed and we all bowed. Then he raised his head from the bowing position and we all raised. Then he went down to prostrate alone with the line which was closest to him, while the rear line remained standing to fight the enemy. When the Prophet finished the prostration and the line that was nearest him stood, the rear line went down to prostrate, then stood. Then the rear line advanced while the forward line went back, then the Prophet ﷺ bowed, and we all bowed. Then he raised his head from the bowing position and we raised. Then he went down to prostrate with the line nearest to him, which was the rear line in the first Rak'ah. The rear line remained standing to fight the enemy. When the Prophet finished the prostration with the line nearest to him, the rear line went down to prostrate, they prostrated, then the Prophet said the Taslim and we all said the Taslim." This was recorded by Muslim in his Sahih.

And with Allâh is the facilitation to do what is right.

Source:
Ibn Baz
Fatawa Islamiyah, Vol. 8 Pages 22-23