Musnad Ahmad 30:1262

Abu al-Nadr and Hasan ibn Musa narrated to us, they said: Warqa’ narrated to us, from Abdullah ibn Dinar, from Sa’id ibn Yasar, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever gives in charity the equivalent of a date from pure earnings—and nothing ascends to Allah except that which is pure—then Allah accepts it with His Right Hand, then He nurtures it for its owner, just as one of you nurtures his foal, until it becomes like the mountain."