The Holy Prophet (S) said: "Yes, the reward of a woman during her pregnancy until childbirth, and unto when she weans the child, is like that of a stationed (soldier guarding the Muslims' borders against the attack of pagan enemies) for the sake of Allah. So, if she dies during that, she will have the rank of a martyr."
Man La Yahduruhul Faqih, vol. 3, p. 561
Imam Amir-ul-Mu'mineen ‘Ali (‘a) said: "No milk is greater in prosperity than the mother's milk for the baby to suck from."
Wasa'il-ush Shi'ah vol. 21, p. 452
The Holy Prophet (S) said: "When a woman conceives with child she will be as a warrior who fasts during the day and keeps vigil at nights praying, and strives with his soul and wealth on the way of Allah. Then, when she delivers, there will be for her such a great reward with Allah that she does not know how magnificent it is. Thereafter, when she suckles (the child), she will have as much reward as freeing a child from Ismail's descendants for every suck. And, when she brings suckling the child to an end, an angel beside her tells her to begin the action again for she has surely been forgiven."
Bihar-ul-Anwar, vol. 104, p. 106
The Holy Prophet (S) said: "There is no milk for a baby better than the milk of its mother."
Mustadrak-ul-Wasa'il, section 48