Praise be to Allah.
If a man has intercourse with his wife, he will be rewarded for that, because he is doing something halaal and avoiding something haraam. This is what is stated in the hadeeth of the Prophet (peace and blessings of Allaah be upon him):
It was narrated from Abu Dharr that some people from among the companions of the Prophet (peace and blessings of Allaah be upon him) said to the Prophet (peace and blessings of Allaah be upon him): “O Messenger of Allaah, the rich people will get more reward. They pray as we pray, and they fast as we fast, but they give in charity from their excess wealth.” He said, “Has not Allaah given you things with which you can give charity? Every tasbeehah (saying ‘Subhaan Allaah (Glory be to Allaah)’) is a charity. Every Takbeerah (saying ‘Allaahu akbar (Allaah is Most Great)’) is a charity. Every Tahmeedah (saying ‘al-hamdu-Lillaah (praise be to Allaah)’ is a charity. Every Tahleelah (saying ‘Laa ilaaha ill-Allaah (there is no god but Allaah)’) is a charity. Enjoining what is good is a charity. Forbidding what is evil is a charity. Having intercourse (with one’s wife) is a charity.” They said, “O Messenger of Allaah, if one of us fulfils his desire, is there reward in that?” He said, “Do you not see that if he does it in a haraam way he will have the burden of sin? So if he does it in a halaal way, he will have a reward for that.”
(Narrated by Muslim, 1674)
Imaam al-Nawawi (may Allaah have mercy on him) said:
“The phrase, ‘Having intercourse is a charity’ – the word bud’ (translated here as ‘having intercourse’) may mean intercourse, or it may refer to the private part itself…
This indicates that permissible actions may become acts of worship, if there is a sincere intention. Intercourse may be an act of worship if the intention behind it is to fulfil the rights of one's wife, to treat her kindly as enjoined by Allaah, to seek a righteous child, to keep oneself or one’s wife chaste, to prevent both partners from looking towards or thinking of haraam things, and other good intentions. ‘O Messenger of Allaah, if one of us fulfils his desire, is there reward in that?’”
(Sharh Muslim, 7/92)
With regard to your comment that the reward is equivalent to seventy naafil prayers”
Perhaps this question about the reward for having intercourse with one’s wife comes from your reading of Imaam Nawawi’s commentary on Saheeh Muslim (Sharh al-Nawawi ‘ala Saheeh Muslim). It is a comment which we will quote below, where the shaykh (al-Nawawi – may Allaah have mercy on him) was discussing the issue of enjoining what is good and saying that it is obligatory. Then he discussed the issue of saying Tasbeeh and dhikr, which he noted is Sunnah. Then he explained that the fard act of worship is equivalent to seventy times the naafil act, and said: they reached this conclusion from a hadeeth… Then at the end of this discussion he referred to the phrase “Having intercourse…”
If this is indeed the case, then you should note that the first comment has nothing to do with what follows it. The phrase “from a hadeeth” means from some hadeeth, but Imaam al-Nawawi does not quote it. When he finishes discussing the issue of fard and naafil acts of worship, and the status of each, he then starts his discussion of the phrase “Having intercourse...” So the confusion has arisen from this juxtaposition.
This is what we think. But if you mean that the reward for fard acts of worship is equivalent to the reward for seventy naafil acts, then al-Nawawi did discuss this point and said that there was a hadeeth about it, but he did not quote it.
We know what al-Nawawi was referring to, and this is what was pointed out by al-Haafiz ibn Hajar (may Allaah have mercy on him).
In Ziyaadaat al-Rawdah, al-Nawawi narrated from the imaam of the Haramayn from some of the scholars that the reward for a fard act ofworship is seventy times more than the reward for a naafil act. Al-Nawawi said: they reached this conclusion from a hadeeth.”
The hadeeth in question was quoted by Imaam al-Nawawi in al-Nihaayah. It is the marfoo’ hadeeth of Salmaan concerning the month of Ramadaan: “Whoever seeks to draw closer to Allaah during (this month) by doing some good deeds will be like one who does an obligatory action at any other time, and one who does an obligatory action (during Ramadaan) will be like one who does seventy obligatory actions at any other time.”
This is a da’eef (weak) hadeeth which was narrated by Ibn Khuzaymah, but he was uncertain as to whether it was sound or not.
(al-Talkhees al-Habeer, 3/118)
What he meant (may Allaah have mercy on him) by saying that Ibn Khuzaymah was uncertain as to whether it was sound or not may be understood more clearly by referring to Saheeh Ibn Khuzaymah (3/191), where he includes it under the chapter heading, Baab Fadaa’il Sharh Ramadaan in sahha al-khabr (Chapter on the virtues of the month of Ramadaan, if the report is saheeh).
And the isnaad of this hadeeth includes ‘Ali ibn Zayd ibn Jad’aan, who is a da’eef (weak) narrator.
And Allaah knows best.