حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ الْقَاسِمَ بْنَ مُحَمَّدٍ، وَنَافِعًا، مَوْلَى عَبْدِ اللَّهِ بْنِ عُمَرَ قَالاَ لاَ اعْتِكَافَ إِلاَّ بِصِيَامٍ بِقَوْلِ اللَّهِ تَبَارَكَ وَتَعَالَى فِي كِتَابِهِ ‏{‏وَكُلُوا وَاشْرَبُوا حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الأَبْيَضُ مِنَ الْخَيْطِ الأَسْوَدِ مِنَ الْفَجْرِ ثُمَّ أَتِمُّوا الصِّيَامَ إِلَى اللَّيْلِ وَلاَ تُبَاشِرُوهُنَّ وَأَنْتُمْ عَاكِفُونَ فِي الْمَسَاجِدِ‏}‏ فَإِنَّمَا ذَكَرَ اللَّهُ الاِعْتِكَافَ مَعَ الصِّيَامِ ‏.‏ قَالَ مَالِكٌ وَعَلَى ذَلِكَ الأَمْرُ عِنْدَنَا أَنَّهُ لاَ اعْتِكَافَ إِلاَّ بِصِيَامٍ ‏.‏

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." Translation Not Available

Book reference: Muwatta Imam Malik Book 19 Hadith 696
Web reference: Muwatta Imam Malik Book 19 Hadith 4