I’tikāf is only in a mosque where congregational prayers are held

بسم الله الرحمن الرحيم

وأنتم عاكفون في المساجد [٢: ١٨٧]

“While you are doing I’tikāf in the mosques” [2: 187]

قال ابن حجر: فعلم ‌من ‌ذكر ‌المساجد أن المراد أن الاعتكاف لا يكون إلا فيها [فتح البارئ (٤/ ٢٧٢)]

Ibn Ḥajar said: “So it is known from the mention of the mosques that what is intended is that Iʿtikāf does not occur except in them.” [Fatḥ al-Bārī (4/272)]

قال مالك فعم الله المساجد كلها. ولم يخصص شيئا منها [موطأ مالك (١١١٣)]

Mālik said: “So Allāh made it general for all mosques, and He did not specify any particular one among them” [Muwaṭṭaʾ Mālik (1113)]

This brings to an important principle which al-Shāfi’i said:

وكذلك ينبغي لمن سمع الحديث أن يقول به ‌على ‌عُمُومِه وجملته حتى يجد دِلالة يفرق بها فيه بينه [الرسالة (٨١٨)]

“And likewise, it is befitting for the one who hears the Ḥadīth to speak according to it based on its generality and its totality, until he finds an indication by which he can make a distinction concerning it.” [Al-Risālah (818)]

Meaning that the default is that if a person wants to specify a condition, he must come with evidence for this is something the people of knowledge have consensus about [cf. al-Ishrāf by Ibn al-Mundhir 4/212]. Therefore, those who condition fasting or certain mosques as a condition for I’tikāf are wrong and against clear evidences.

عن الزهري قال: لا اعتكاف إلا في مسجد جماعة يجمع فيه [ابن أبي شيبة (٩٩٣٢) وسنده صحيح]

Al-Zuhrī said: “There is no Iʿtikāf except in a congregational masjid in which congregational prayers are held.” [Ibn Abī Shaybah (9932); Isnād Ṣaḥīḥ]

شعبة قال: سألت الحكم وحمادًا عن الاعتكاف (فقالا): لا يعتكف إلا في مسجد يجمعون فيه [ابن أبي شيبة (٩٩٣٣) وسنده صحيح]

Shuʿbah said: I asked al-Ḥakam and Ḥammād about Iʿtikāf, so they said: “One does not perform Iʿtikāf except in a masjid where they gather for congregational prayers.” [Ibn Abī Shaybah (9933); Isnād Ṣaḥīḥ]

عن أبي جعفر قال: لا اعتكاف إلا في مسجد يجمع فيه [ابن أبي شيبة (٩٩٣٤) وسنده صحيح]

Abū Jaʿfar (al-Ṣādiq) said: “There is no Iʿtikāf except in a masjid in which congregational prayers are held.” [Ibn Abī Shaybah (9934); Isnād Ṣaḥīḥ]

عروة بن الزبير قال: لا اعتكاف إلا في مسجد جماعة [ابن أبي شيبة (٩٩٣٥) وسنده صحيح]

ʿUrwah bin al-Zubayr said: “There is no Iʿtikāf except in a congregational masjid.” [Ibn Abī Shaybah (9935); Isnād Ṣaḥīḥ]

All of these narrations have been authenticated by shaykh Amanpuri. Another benefit derived from this is that women can’t do I’tikāf in their house [cf. Sharḥ Ṣaḥīḥ Muslim 8/68]