Al-Kāfi - Volume 4 > ‘1‘tikaf is not Valid without Fasting
Hadith #2

2ـ مُحَمَّدُ بْنُ يَحْيَى عَنْ مُحَمَّدِ بْنِ الْحُسَيْنِ عَنْ صَفْوَانَ بْنِ يَحْيَى عَنِ الْعَلاءِ بْنِ رَزِينٍ عَنْ مُحَمَّدِ بْنِ مُسْلِمٍ قَالَ قَالَ أَبُو عَبْدِ الله (عَلَيْهِ السَّلاَم) لا اعْتِكَافَ إِلاَّ بِصَوْمٍ.

2. Muhammad ibn Yahya has narrated from Muhammad ibn al-Husayn, from Safwan ibn Yahya from al-‘Ala’ ibn Razin from Muhammad ibn Muslim who has said the following: “Abu ‘Abd Allah, ‘Alayhi al-Salam, has said, ‘Without fast ’I‘tikaf is not valid.’”