(6) Zakat
كتاب الزكاة
(3b) Chapter: Sadaqa when Ramadan ends - Section 2
(2) باب صدقة الفطر - الفصل الثاني
Mishkat al-Masabih 1818 
He said that God’s messenger prescribed the zakat relating to the breaking of the fast as a purification of the fasting from empty and obscene talk and as food for the poor. Abu Dawud transmitted it.
وَعَن ابْنِ عَبَّاسٍ قَالَ: فَرَضَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ زَكَاةَ الْفِطْرِ طُهْرَ الصِّيَامِ مِنَ اللَّغْوِ وَالرَّفَثِ وَطُعْمَةً لِلْمَسَاكِينِ. رَوَاهُ أَبُو دَاوُد
Reference : Mishkat al-Masabih 1818
In-book reference : Book 6, Hadith 47