Sahih Muslim Book 8, Hadith Number 3422.
Chapter: One suckling or two do not make marriage unlawful.
‘Amra reported that she beard ‘Aisha (Allah he pleased with her) discussing fosterage which (makes marriage) unlawful; and she (‘Aisha) said: There was revealed in the Holy Qur’an ten clear sucklings, and then five clear (sucklings).