Masjid al-Fath
Location
Masjid al-Fath is situated on the Western side of the Sal' mountain, at the historical site where the Messenger of Allah (peace and blessings be upon him) made supplications against the confederate armies during the Battle of the Trench. His prayers were answered by Allah Ta'ala, who sent a vehement wind that overturned the enemy's cooking vessels and tents, ultimately causing their retreat.
Names
It is called Masjid al-Fath because Allah Taala revealed in it revelation of victory. Imam al-Haytami said:
"It was named as such due to his (Allah bless him and give him peace) saying after he had prayed and supplicated: Glad tidings of a victory from Allah and His support." [Hashiyah al-Haytami ala al-Idah p.458]
It is not - as some mistakenly believe - because Surah al-Fath was revealed in it, rather the scholars mention that Surah al-Fath was revealed between Makka and Madina regarding Hudaybiyyah.
The mosque is also referred to as Masjid al-Ahzab as the Prophet (Allah bless him and give him peace) supplicated therein against the Ahzab (confederate forces) during the Battle of the Trench as narrated in Sahih al-Bukhari:
"O Allah vanquish the Ahzab (confederate armies)."
It has also been referred to as Masjid al-Ala due to its location upon a higher elevation on the mountain. [Tarikh Madina of Ibn Shabbah]
Architectural History
Originally built by Umar bin Abd al-Aziz, the mosque consisted of a single covered area supported by three pillars. It experienced disrepair over time and was renovated in 575 Hijri by Amir Sayf al-Din al-Hussain Ibn Abi al-Hayja'. Ayyub Sabri recorded another renovation in 1853 (1270H) during Sultan Abd al-Majid's reign, and the structure that stands today stems from this period, along with further enhancements during the Saudi era.
The Prophetic Supplication
Jabir ibn ‘Abdullah said,
‘The Prophet ﷺ supplicated in the Mosque of Victory (Fath) thrice. On Monday, Tuesday and Wednesday. The supplication was answered for him ﷺ between the two prayers on Wednesday. Delight was apparent on his ﷺ face.’ Jabir said, ‘Whenever anything important or formidable occurred I sought out that time and supplicated in it, and I was answered.’
Al-Haythami said: It was narrated by Ahmad and al-Bazzar and Ahmad's narrators are trustworthy.
The Specific Location of Prayer and Supplication
Some of the early Muslim specificed the exact location the Messenger of Allah (Allah bless him and give him peace) prayed and supplicated on this mountain. Jafar Ibn Muhammad narrated from his father that:
"The Prophet (Allah bless him and give him peace) entered Masjid al-Fath, he took a step and then a second step, he then raised his hands and began to supplicate, and he supplicated for a lengthy period of time, and then left." [Wafa' al-Wafa]
Abu Ghassan said: It is mentioned that the location where the Messenger of Allah (Allah bless him and give him peace) supplicated on the mountain is today the location of the middle pillar..." [Tarikh al-Madina of Ibn Shabbah]
Yahya bin Yahya said:
"I entered Masjid al-Fath with Hussain bin Abdullah, when he reached the middle pilar of the Masjid he said: This is the spot where the Messenger of Allah (Allah bless him and give him peace) prayed and supplicated against the Ahzab."