Sahih Muslim
The Book of Marriage (Kitab Al-Nikah)
Book Number: 008 / Hadith Number: 3376
Abu Sa'id al-Khudri (Allah be pleased with him) reported that Allah's Apostle (may peace be upon him) was asked about 'azl, whereupon he said: 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.