From Abu Dharr, may Allah be pleased with him, also, there is that, “Some people from the Companions of the Messenger of Allah said to the Prophet, may Allah bless him and grant him peace, ‘Messenger of Allah, the people of great wealth have gone off with the rewards. They pray as we pray, fast as we fast, and pay the zakah with the excess of their properties’. He said, ‘Has Allah not given you that with which you can give sadaqah? Truly, in every glorification there is sadaqah, in every magnification (takbir) there is sadaqah, every praise is sadaqah, in every “There is no god but Allah” there is sadaqah, in every command to the good there is sadaqah, in every forbidding of the wrong there is sadaqah and in every act of sexual intercourse by any of you is sadaqah’. They said, ‘Messenger of Allah, can one of us approach his appetite and have a reward for it?’ He said, ‘What do you think, if he had put it into something haram would there not have been a wrong [written] against him? Similarly if he puts it in the halal there is a reward for him’.” Muslim narrated it.
His saying, “They said, ‘Messenger of Allah, can one of us approach his appetite and have a reward for it?’ He said, ‘What do you think, if he had put it into something haram would there not have been a wrong [written] against him?” Know that the appetite for sexual intercourse is one which the prophets and people of right action love, they say for those benefits of the deen and world which are in it: lowering the eye, breaking the appetite so that one doesn’t approach adultery, and gaining children by whom the populating the world and the increase of the ummah until the Day of Resurrection are made complete . They say that involvement with all other types of appetite hardens the heart except for this one, for it softens the heart.
 Yatasaddaqoun, from sadaqa: it is clear that the sadaqah referred to here refers first to the obligatory zakah, because the wealthy are enabled to pay zakah and more, and second to optional acts of generosity, since the poor have the option of extra voluntary acts, but do not attain the level of having to pay zakah, and Allah knows best.
 Assumed in this understanding is that multiple-wife families are sunnah, but that it is permissible for a man to have only one wife. This is the contrary of the modern understanding, fabricated in the last century by scholars intent of proving themselves acceptable to the monogamous (but highly adulterous) Jews and Christians, that multiple-wife families are only acceptable in extraordinary situations of Jihad and the like.
(Imam Muhyi al-Din Abu Zakariya Yahya bin Sharaf al-Nawawi, Arba`een, translated by Abdassamad Clarke and published by Ta-Ha Publishers Ltd., as The Complete Forty Hadith). In commentary of hadith no.25, p.100 -102)