"...and your suckling mothers who suckled you and your sisters from suckling..." [An-Nisaa:23]
It is narrated in the two Saheeh collections (Al-Bukhaari and Muslim) that the Prophet, (sallAllaahu 'alayhi wa sallam) said:
"It becomes forbidden by suckling those relations that are forbidden by kinship."
Furthermore, his saying:
"It becomes forbidden by suckling those relations that are forbidden by birth."
Narrated by the group (1).
The meaning of breast-feeding, linguistically, is: "The suckling of milk from the breast or the drinking of it."
It's definition according to the Sharee'ah is: "The suckling or drinking etc. of the milk induced by pregnancy by a child younger than two years old."