Sahih Muslim Book 5, Hadith Number 2198.

Abu Dharr reported: some of the people from among the Companions of the Apostle of Allah (may peace be upon him) said to him: Messenger of Allah, the rich have taken away (air the) reward. They observe prayer as we do; they keep the fasts as we keep, and tray give Sadaqa out of their surplus riches. Upon this he (the Holy Prophet) said: Has Allah not prescribed for you (a course) by following which you can (also) do sadaqa? In every declaration of the glorification of Allah (i.e. saying Subhan Allah) there is a Sadaqa, and every Takbir (i.e. saying Allah-O-Akbar) is a sadaqa, and every praise of His (saying al-Hamdu Lillah) is a Sadaqa and every declaration that He is One (La illha ill-Allah) is a sadaqa, and enjoining of good is a sadaqa, and forbidding of that which is evil is a Sadaqa, and in man’s sexual Intercourse (with his wife,) there is a Sadaqa. They (the Companions) said: Messenger of Allah, is there reward for him who satisfies his sexual passion among us? He said: Tell me, if he were to devote it to something forbidden, would it not be a sin on his part? Similarly, if he were to devote it to something lawful, he should have a reward.