Mishkat al-Masabih 6 Zakat كتاب الزكاة 47

وَعَن ابْنِ عَبَّاسٍ قَالَ: فَرَضَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ زَكَاةَ الْفِطْرِ طُهْرَ الصِّيَامِ مِنَ اللَّغْوِ وَالرَّفَثِ وَطُعْمَةً لِلْمَسَاكِينِ. رَوَاهُ أَبُو دَاوُد

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.