Muwatta Malik 626

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم فَرَضَ زَكَاةَ الْفِطْرِ مِنْ رَمَضَانَ عَلَى النَّاسِ صَاعًا مِنْ تَمْرٍ أَوْ صَاعًا مِنْ شَعِيرٍ عَلَى كُلِّ حُرٍّ أَوْ عَبْدٍ ذَكَرٍ أَوْ أُنْثَى مِنَ الْمُسْلِمِينَ ‏.‏

Yahya related to me from Malik from Nafi from Abdullah ibn Umar that the Messenger of Allah, may Allah bless him and grant him peace, made the zakat of breaking the fast at the end of Ramadan obligatory on every muslim, whether freeman or slave, male or female, and stipulated it as a sa' of dates or a sa' of barley

Reference

Muwatta Malik 626

In-book

Book 17, Hadith 49