Allah, Glorified be His praise, says: "And mothers shall breastfeed their children for two full years, (this is) for the one who wishes to complete the nursing; and upon the one to whom the child is born (i.e. the father) is their provision and their clothing according to what is customary." [Surah al-Baqarah: 233]
And He said: "Then if they breastfeed (the child) for you, give them their payment." [Surah al-Talaq: 6]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النفقات / Q15733