Hadith 25, 40 Hadith an-Nawawi
Also on the authority of Abu Dharr (may Allah be pleased with him):
Some people from amongst the Companions of the Messenger of Allah (peace and blessings of Allah be upon him) said to the Prophet (peace and blessings of Allah be upon him), "O Messenger of Allah, the affluent have made off with the rewards; they pray as we pray, they fast as we fast, and they give [much] in charity by virtue of their wealth." He (peace and blessings of Allah be upon him) said, "Has not Allah made things for you to give in charity? Truly every tasbeehah [saying: 'subhan-Allah'] is a charity, and every takbeerah [saying: 'Allahu akbar'] is a charity, and every tahmeedah [saying: 'al-hamdu lillah'] is a charity, and every tahleelah [saying: 'laa ilaha illAllah'] is a charity. And commanding the good is a charity, and forbidding an evil is a charity, and in the bud`i [sexual act] of each one of you there is a charity."

They said, "O Messenger of Allah, when one of us fulfils his carnal desire will he have some reward for that?" He (peace and blessings of Allah be upon him) said, "Do you not see that if he were to act upon it [his desire] in an unlawful manner then he would be deserving of punishment? Likewise, if he were to act upon it in a lawful manner then he will be deserving of a reward." [Muslim]
عَنْ أَبِي ذَرٍّ رَضِيَ اللهُ عَنْهُ أَيْضًا، "أَنَّ نَاسًا مِنْ أَصْحَابِ رَسُولِ اللَّهِ صلى الله عليه و سلم قَالُوا لِلنَّبِيِّ صلى الله عليه و سلم يَا رَسُولَ اللَّهِ ذَهَبَ أَهْلُ الدُّثُورِ بِالْأُجُورِ؛ يُصَلُّونَ كَمَا نُصَلِّي، وَيَصُومُونَ كَمَا نَصُومُ، وَيَتَصَدَّقُونَ بِفُضُولِ أَمْوَالِهِمْ. قَالَ: أَوَلَيْسَ قَدْ جَعَلَ اللَّهُ لَكُمْ مَا تَصَّدَّقُونَ؟ إنَّ بِكُلِّ تَسْبِيحَةٍ صَدَقَةً، وَكُلِّ تَكْبِيرَةٍ صَدَقَةً، وَكُلِّ تَحْمِيدَةٍ صَدَقَةً، وَكُلِّ تَهْلِيلَةٍ صَدَقَةً، وَأَمْرٌ بِمَعْرُوفٍ صَدَقَةٌ، وَنَهْيٌ عَنْ مُنْكَرٍ صَدَقَةٌ، وَفِي بُضْعِ أَحَدِكُمْ صَدَقَةٌ. قَالُوا: يَا رَسُولَ اللَّهِ أَيَأْتِي أَحَدُنَا شَهْوَتَهُ وَيَكُونُ لَهُ فِيهَا أَجْرٌ؟ قَالَ: أَرَأَيْتُمْ لَوْ وَضَعَهَا فِي حَرَامٍ أَكَانَ عَلَيْهِ وِزْرٌ؟ فَكَذَلِكَ إذَا وَضَعَهَا فِي الْحَلَالِ، كَانَ لَهُ أَجْرٌ". 
[رَوَاهُ مُسْلِمٌ].