Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.152.

It is good for the person performing i’tikaf to fast, but he is not under any obligation to do so. Al-Bukhari records from Ibn ‘Umar that ‘Umar said: “O Messenger of Allah, during the days of ignorance I vowed to perform i’tikaf one night in the mosque at Makkah. The Prophet said: ‘Fulfill your vow.'” This statement of the Prophet, upon whom be peace, shows that fasting is not a condition for i’tikaf; otherwise, performing i’tikaf at night would not be valid. Sa’id ibn Mansur records that Abu Sahl said: “One of my wives was to perform i’tikaf, so I asked ‘Umar ibn ‘Abdulaziz about it. He said: ‘She need not fast, unless she imposes it upon herself.’ Az-Zuhri said: ‘There is no i’tikaf save while fasting.’ ‘Umar asked: ‘Is this from the Prophet?’ Az-Zuhri answered, ‘No.’ ‘Umar asked, ‘From Abu Bakr?’ Az-Zuhri said,’No.’ ‘Umar asked [again], ‘From ‘Umar [ibn al-Khattab]?’ Az-Zuhri said, ‘No.’ ‘Umar said: ‘I suspect he said it from ‘Uthman?’ Az-Zuhri said, ‘No.’ I [Abu Sahl] left them and met ‘Ata and Tawus and asked them about it. Tawus said: ‘A person would see that he did not have to fast unless he imposed it on himself.'”

Al-Khattabi acknowledges [the differences on the issue]: “There is a difference of opinion among the people on this point.”

Al-Hassan al-Basri holds: “Performing i’tikaf without fasting suffices. That is also the opinion of ash-Shaf’i.”

‘Ali and Ibn Mas’ud maintain: “If one wishes, one may fast and if one does not wish to, one does not have to.”

Al-Auza’i and Malik hold: “There is no i’tikaf without fasting, and that is the conclusion of the people of opinion. That has been related from Ibn ‘Umar, Ibn ‘Abbas, and ‘Aishah, and it is the opinion of Sa’eed ibn al-Musayyeb, ‘Urwah ibn az-Zubair, and az-Zuhri.”