Al-Kāfi - Volume 4 > Minimum Limits of ’1‘tikaf
Hadith #2

2ـ أَحْمَدُ بْنُ مُحَمَّدٍ عَنِ ابْنِ مَحْبُوبٍ عَنْ أَبِي أَيُّوبَ عَنْ أَبِي بَصِيرٍ عَنْ أَبِي عَبْدِ الله (عَلَيْهِ السَّلاَم) قَالَ لا يَكُونُ الاعْتِكَافُ أَقَلَّ مِنْ ثَلاثَةِ أَيَّامٍ وَمَنِ اعْتَكَفَ صَامَ وَيَنْبَغِي لِلْمُعْتَكِفِ إِذَا اعْتَكَفَ أَنْ يَشْتَرِطَ كَمَا يَشْتَرِطُ الَّذِي يُحْرِمُ.

2. Ahmad ibn Muhammad from has narrated from ibn Mahbub from abu Ayyub from abu Basir who has said the following: “Abu ‘Abd Allah, ‘Alayhi al-Salam, has said, ‘An Ttikaf for less than three days is not valid. One who is in Ttikaf must also fast and it is proper for one in Ttikaf to set a condition as a person in the state of ’Ihram does.’”