Mishkat al-Masabih 6 Zakat كتاب الزكاة 153

وَعَنْ أَبِي أُمَامَةَ قَالَ: قَالَ أَبُو ذَرٍّ: يَا نَبِيَّ اللَّهِ أَرَأَيْتَ الصَّدَقَةُ مَاذَا هِيَ؟ قَالَ: «أَضْعَافٌ مُضَاعَفَةٌ وَعِنْدَ اللَّهِ الْمَزِيدُ» . رَوَاهُ أَحْمد

Abu Umama told of Abu Dharr asking God’s Prophet to tell him what the reward for sadaqa would be, and receiving the reply, “Many times as much, and more still with God.” Ahmad transmitted it.