Sahih Muslim
The Book of Marriage (Kitab Al-Nikah)
Book Number: 008 / Hadith Number: 3422
'Amra reported that she beard 'A'isha (Allah he pleased with her) discussing fosterage which (makes marriage) unlawful; and she ('A'isha) said: There was revealed in the Holy Qur'an ten clear sucklings, and then five clear (sucklings).