Al-Kāfi - Volume 4 > ‘1‘tikaf is not Valid without Fasting
Hadith #3

3ـ عَلِيُّ بْنُ إِبْرَاهِيمَ عَنْ أَبِيهِ عَنِ ابْنِ أَبِي عُمَيْرٍ عَنْ حَمَّادٍ عَنِ الْحَلَبِيِّ عَنْ أَبِي عَبْدِ الله (عَلَيْهِ السَّلاَم) قَالَ لا اعْتِكَافَ إِلاَّ بِصَوْمٍ فِي الْمَسْجِدِ الْجَامِعِ.

3. Ali ibn Ibrahim has narrated from his father from ibn abu ‘ Umayr from Hammad from al-Halabiy who has said the following: “Abu ‘Abd Allah, ‘Alayhi al-Salam, has said, ‘Without fast and Jami‘ (central) Masjid Ttikaf is not valid.’”