Al-Kāfi

Hadith 680
itikaffasting
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 (a.s.), has said, ‘Without fast ’I‘tikaf is not valid.’”
Source: Shaykh Muḥammad b. Yaʿqūb al-Kulaynī