Musnad Ahmad 937:258

Yazid narrated to us, Hisham informed us, from Wasil, from Yahya ibn Aqil, from Yahya ibn Ya'mar, from Abu Dharr, from the Prophet, peace and blessings of Allah be upon him, who said: "Every morning, upon every joint of the son of Adam, there is a charity." Then he said: "Your removing of harm from the road is a charity, your greeting of people is a charity, your enjoining of good is a charity, your forbidding of evil is a charity, and your having intercourse with your spouse is a charity." We said: "O Messenger of Allah, does a man satisfy his desire and have it be a charity for him?" He said: "Yes. Do you see, if he were to place that desire in what Allah has forbidden for him, would he not have a sin upon him?" We said: "Yes." He said: "Then, if he places it in what Allah has made lawful, it is a charity." He mentioned other things as charity, and charity. Then he said: "And two rak'ahs of Duha (forenoon prayer) suffice for all of that."