Sunan Abi Dawud Book 5, Hadith 37
While we were present with Abu Dharr, he said: In the morning, alms are due for him, ever fast is alms, every pilgrimage is alms, every utterance of "Glory to be Allah" is alms, every utterance of "Allah is most great" is alms, every utterance of "Praise be to Allah" is alms. The Messenger of Allah (ﷺ) recounted all such good works. He then said: Two rak'ahs which one prays in the Duha serve instead of that.
The Prophet (ﷺ) as saying: In the morning alms are due for every bone in man's body. His salutation to everyone he meets is alms, his enjoining good is alms, his forbidding what is evil is alms, the removal of harmful thing from the way is alms, to have sexual intercourse with one's wife if alms, and two rak'ahs which one prays in the Duha serve instead of that.
Abu Dawud said: The tradition narrated by 'Abbad is more perfect (than the version narrated by Musaddad). Musaddad did not mention in his version "the command (of good) and the prohibition (of evil)". Instead, he added in his version saying: "Such and such." Ibn Ma'na added in his version: "They (the people) said: Messenger of Allah, how is that one of us fulfills his desire and still there are alms for him (i.e. is rewarded)? He replied: What do you think if you had unlawful sexual intercourse, would he not have been a sinner ?
The Prophet (ﷺ) said: If anyone sits in his place of prayer when he finishes the dawn prayer till he prays the two rak'ahs of the forenoon, saying nothing but what is good, his sins will be forgiven even if they are more than the foam of the sea.