Yahya related to me from Malik that he had heard that al-Qasimibn Muhammad and Nafi, the mawla of Abdullah ibn Umar said, "Youcannot do itikaf unless you are fasting, because of what Allah, theBlessed and Exalted, says in His Book, 'And eat and drink until thewhite thread becomes clear to you from the black thread of dawn, thencomplete the fast until night-time, and do not have intercourse withthem while you are doing itikaf in mosques,' (Sura 2 ayat 187). Allahonly mentions itikaf together with fasting."Malik said,"That is what we go by here."
USC-MSA web (English) reference: Book 19, Hadith 4
Arabic reference: Book 19, Hadith 696