Al-Kāfi

Hadith 691
itikaffasting
5ـ عِدَّةٌ مِنْ أَصْحَابِنَا عَنْ سَهْلِ بْنِ زِيَادٍ عَنْ أَحْمَدَ بْنِ مُحَمَّدٍ عَنْ دَاوُدَ بْنِ سِرْحَانَ قَالَ بَدَأَنِي أَبُو عَبْدِ الله (عَلَيْهِ السَّلاَم) مِنْ غَيْرِ أَنْ أَسْأَلَهُ فَقَالَ الاعْتِكَافُ ثَلاثَةُ أَيَّامٍ يَعْنِي السُّنَّةَ إِنْ شَاءَ الله.
5. A number of our people have narrated from Sahl ibn Ziyad from Ahmad ibn Muhammad from Dawud ibn Sarhan who has said the following: “Abu ‘Abd Allah (a.s.), once initiated speaking without my asking anything, ‘’Ttikaf consists of three days, meaning, according to the Sunnah, by the will of Allah.’”
Source: Shaykh Muḥammad b. Yaʿqūb al-Kulaynī