Ibn 'Abbas (RAA) narrated that The Messenger of Allah (ﷺ) 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 (ﷺ). (IT)
English reference (IT): Book 5, Hadith 723 (IT)
Arabic reference (IT): Book 5, Hadith 703 (IT)