Does vowing I'tikaf in a specific mosque necessitate fulfilling it only in that mosque?
General Chapter
Al-Mughni
Book of I'tikaf
Primary text
No mosque is designated by a vow of I'tikaf unless it is one of the Three Sacred Mosques: Al-Masjid al-Haram, the Prophet's Mosque (Al-Masjid an-Nabawi), or Al-Aqsa Mosque, based on the Hadith restricting travel ('shedding of loads') to these three. If a non-sacred mosque is specified, one is bound to proceed there and requires the severing of travel ties to fulfill the vow. These three became designated because Allah did not specify a location for worship, but a specific text designated these three due to their superior merit.
Supporting text
Al-Shafi'i, in his sounder opinion, agrees. In his other opinion, he argued that Al-Aqsa Mosque is not designated, based on the Hadith stating prayer in the Prophet's Mosque is better than a thousand prayers elsewhere, except in Al-Masjid al-Haram. This implies equality outside of those two. However, the primary view is that since Al-Aqsa is one of the mosques to which travel is permitted, it becomes designated by the vow, similar to the Prophet's Mosque.