Sahih Muslim
The Book of Marriage (Kitab Al-Nikah)
Book Number: 008 / Hadith Number: 3374
Abu Sa'id al-Khudri (Allah be pleased with him) (was asked if he had heard it himself), to which he said: Yes. (I heard) Allah's Apostle (may peace be upon him) as saying: There is no harm if you do not practise it, for it (the birth of the child) is something ordained (by Allah).