Musnad Ahmad 30:2437

Yahya narrated to us, from Ibn ‘Ajlan, who said: Sa‘id ibn Yasar narrated to me, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "There is no Muslim who gives charity from pure earnings—and nothing ascends to the heaven except that which is pure—but that it is as if he places it in the palm of the Most Merciful (Mighty and Majestic is He). So He nurtures it just as one of you nurtures his foal or his young camel, until the date becomes like a great mountain."