Muwatta Malik, Book 58, Hadith 1844 (In-Book)

Malik related to me from Yahya ibn Said from Abu'l-Hubab Said ibn Yasar that the Messenger of Allah, may Allah bless him and grant him peace, said, "Whoever gives sadaqa from good earning - and Allah only accepts the good - it is as if he placed it in the palm of the Merciful to raise it, as one of you raises his foal or young camel until it is like the mountain "

حَدَّثَنِي مَالِكٌ، عَنْ يَحْيَى بْنِ سَعِيدٍ، عَنْ أَبِي الْحُبَابِ، سَعِيدِ بْنِ يَسَارٍ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ مَنْ تَصَدَّقَ بِصَدَقَةٍ مِنْ كَسْبٍ طَيِّبٍ - وَلاَ يَقْبَلُ اللَّهُ إِلاَّ طَيِّبًا - كَانَ إِنَّمَا يَضَعُهَا فِي كَفِّ الرَّحْمَنِ يُرَبِّيهَا كَمَا يُرَبِّي أَحَدُكُمْ فَلُوَّهُ أَوْ فَصِيلَهُ حَتَّى تَكُونَ مِثْلَ الْجَبَلِ ‏" ‏ ‏.‏

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also referenced under: Book 58, Hadith 1 (USC-MSA)