Sayyidah Aishah (may Allah be pleased with her) narrates that she said: "There is no i'tikaf without fasting."
(Al-Zuhri narrated a hadith from Urwah from Sayyidah Aishah (may Allah be pleased with her), at the end of which it says that the sunnah for one who performs i'tikaf is that he fasts.)
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8579