It is narrated from Abdullah (bin Mas'ud) (may Allah be pleased with him): "Fosterage is only what takes place within the two years, and what grows the flesh and strengthens the bones." Yahya bin Sa'id narrated it with the same meaning from Abdullah bin Mas'ud (may Allah be pleased with him) as mursal, concerning the two years.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرضاع / 15665