Bulugh al-Maram, Book 5, Hadith 723 (English) ⓘ

Ibn ’Abbas (RAA) narrated that The Messenger of Allah said: The one who is performing I'tikaf does not have to fast unless he obliges himself to do so.” Related by Ad-Daraqutni and al-Hakim, and most probably it is also the saying of Ibn 'Abbas, i.e. Mawquf and not of the sayings of the Prophet.

وَعَنْ اِبْنِ عَبَّاسٍ رَضِيَ اَللَّهُ عَنْهُمَا; أَنَّ اَلنَّبِيَّ ‏- صلى الله عليه وسلم ‏-قَالَ: { لَيْسَ عَلَى اَلْمُعْتَكِفِ صِيَامٌ إِلَّا أَنْ يَجْعَلَهُ عَلَى نَفْسِهِ } رَوَاهُ اَلدَّارَقُطْنِيُّ وَالْحَاكِمُ, وَالرَّاجِحُ وَقْفُهُ أَيْضًا 1‏.‏ ‏1 ‏- ضعيف.‏ رواه الدارقطني ( 2 / 199 / 3 )‏، والحاكم ( 1 / 439 )‏.‏