‘’Having intercourse (with one’s wife) is a charity.”
The Messenger (sallal-laahu-alayhi-wasallam) said: ”Having intercourse (with one’s wife) is a charity.” 
In this there is: Perfect enjoyment; Kindness towards a beloved wife; seeking reward; obtainment of reward equivalent to giving Sadaqah; exhilaration of the soul; expelling bad thoughts from her; lighten the soul; expel her strain; lightening her body and balancing her temperament. If this happens in a good way, with gentleness and a lot of passion, complete desire and seeking reward, then there will not be any enjoyment equal to it….
 [Reported by Muslim. Number 1674]
Abridged and slightly paraphrased. See: Rawdatul Muhibbeen Wa Nuzhatul Mush-taaqeen of Imaam Ibnul Qayyim. Page 187]