Allah, the Blessed and Exalted, says: "Mothers shall nurse their children for two full years; (this is) for one who wishes to complete the nursing." [Surah al-Baqarah: 233]
Sufyan narrated to us from Abdullah bin Dinar, who narrates from Ibn Umar (may Allah be pleased with them both) that he said: I heard Umar (may Allah be pleased with him) saying: There is no fosterage except in the first two years of childhood.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرضاع / 15663