وَعَنْ اِبْنِ عَبَّاسٍ رَضِيَ اَللَّهُ عَنْهُمَا;
أَنَّ اَلنَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «لَيْسَ عَلَى اَلْمُعْتَكِفِ صِيَامٌ إِلَّا أَنْ يَجْعَلَهُ عَلَى نَفْسِهِ»
The one who is performing Itikaf 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 (sallallahu alayhi wa sallam).
Bulugh al-Maram 811, Book 5, Fasting, Hadith 54. 1 min read