It is narrated from Sayyidah Aishah (may Allah be pleased with her) that verses of ten known sucklings were revealed in the Qur'an, which made (marriage) unlawful; then they were reduced to five known sucklings, which make it unlawful. No one used to come in to Sayyidah Aishah (may Allah be pleased with her) except one who had completed five sucklings.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرضاع / 15621