He who feeds a fasting person will earn the same reward as him (the fasting person), without diminishing anything from the reward of the fasting person [Ibn Maajah]

"«مَنْ فَطَّرَ صائمًا، كان له مِثْلُ أجْرِهِ، غَيْرَ أنَّهُ لاَ يُنْقَصُ مِنْ أجْرِ الصَّائِمِ شَيْءٌ». [صحيح.] - [رواه الترمذي"