The Messenger of Allah ﷺ said, "Whoever gives in charity the equivalent of a date from lawful earnings, and nothing ascends to Allah except what is wholesome, Allah accepts it with His right hand and then nurtures it for the giver just as one of you nurtures his foal, until it becomes like a mountain."
حدثنا أبو النضر وحسن بن موسى قالا ثنا ورقاء عن عبد الله بن دينار عن سعيد بن يسار
قال رسول الله ﷺ من تصدق بعدل تمرة من كسب طيب ولا يصعد إلى الله إلا الطيب فإن الله يقبلها بيمينه ثم يربيها لصاحبها كما يربي أحدكم فلوه حتى تكون مثل الجبل
Its isnad is sahih. Narrated by Bukhari and Tirmidhi. "Adl tamrah" means the value of a date, and "good earning" means what is lawful. The meaning of "He nurtures it for its owner" is that Allah multiplies the reward or increases the quantity. "Al-faluww" (with fatha on the fa, damma on the lam, and fatha on the doubled waw) refers to the foal once it has grown large enough to need rearing by someone other than its mother.Tirmidhi narrated it as, "until the morsel becomes like Uhud." The example is drawn from the foal because it grows with a clear, visible increase, and because charity is the fruit of one's labor. When a servant gives charity from good earnings, Allah keeps multiplying the reward for him, until the relation between what he gave and what it becomes is like the relation between a date and a mountain.