There is no harm if you do not do that, for it (the birth of the child) is something ordained. Muhammad (one of the narrators) said: (The words) La 'alaykum (there is no harm) implies its Prohibition.
Sahih Muslim Book 16, Hadith 151
← Previous Hadith This hadith is reported on the authority of Abu Sa'id with the same chain of transmitters but with a slight variation (of words).

Sahih Muslim Book 16, Hadith 150 Next Hadith → Why do you practise it? They said: There is a man whose wife has to suckle the child, and if that person has a sexual intercourse with her (she may conceive) which he does not like, and there is another person who has a slave-girl and he has a sexual intercourse with her, but he does not like her to have conception so that she may not become Umm Walad, whereupon he (the Holy Prophet) said: There is no harm if you do not do that, for that (the birth of the child) is something pre- ordained. Ibn 'Aun said: I made a mention of this hadith to Hasan, and he said: By Allah, (it seems) as if there is upbraiding in it (for 'azl).

Sahih Muslim Book 16, Hadith 152