The Suckling Imperative
According to Ya'qubi, an Arab historian and geographer, Muhammad once said that his first wet nurse was the nastiest of women and that she would burn for all eternity in the fires of Hell. To quench her thirst she would only have the small amount of milk she allowed him to suckle in the days following his birth.
The Prophet also reported having a vision of Hell where women who had not suckled their children had snakes biting at their breast.
The revelations on how long a mother or a wet-nurse must suckle a child and what the parents must do if they choose to shorten the weaning period decreed by Allah may have had something to do with God’s Messenger’s awareness that he had not been adequately breastfed when he was a baby, and Allah wishing that such a calamity be visited on as few children as possible.
2:233 Mothers shall suckle their children for two whole years; [that is] for those who wish to complete the suckling. Those to whom the children are born (the fathers) shall maintain and clothe them kindly. No soul is charged beyond its capacity. No mother should suffer on account of her child and he to whom a child is born should not suffer on account of his child. The same [duties (the maintenance and clothing of divorced women)] devolve upon the [father’s] heir (if the heir is a child and has a guardian the latter would be charged with those duties). But they commit no offence if by mutual agreement and following consultation they choose to wean the child. You also commit no offence if you engage wet-nurses, provided that you give them what you promised to give kindly. Fear Allah and know that Allah has knowledge of what you do.
If the weaning period is included in the suckling period (24 months) then Allah, in the following verse where He decrees that the gestation and weaning period combined total thirty months, has underestimated the gestation period of the human fetus by three months (30 - 24 = 6 months gestation time).
46:15 We have commanded man to be kind to his parents; his mother bore him painfully and delivered him painfully, his gestation and weaning totalling thirty months. When he is fully grown and turns forty, he will say: “Lord, inspire me to be thankful for the favour, with which You have favoured me and favoured my parents; and to do a righteous deed, well-pleasing to You. Grant me righteousness in my progeny; I have truly repented to You and one of those who submit.
Breast milk could sour a marriage.
Narrated Abdullah bin Abu Mulaika from 'Uqba bin Al-Harith:
Uqba married the daughter of Abu Ihab bin Aziz, and then a woman came and said, "I suckled 'Uqba and his wife."
Uqba said to her, "I do not know that you have suckled me, and you did not inform me." He then sent someone to the house of Abu Ihab to enquire about that but they did not know that she had suckled their daughter.
Then Uqba went to the Prophet in Medina and asked him about it.
The Prophet said to him, "How (can you keep your wife) after it has been said (that both of you were suckled by the same woman)?"
So, he divorced her and she was married to another (husband).
Making a Male a Relation by Suckling Him
For the Prophet, breasts were not only utilitarian attributes of a female, fun to play with*, but also what they contained possessed almost magical properties. For example, God's Messenger said that a woman who suckled an adult male who was not a close relation made marriage to her not only impossible for him but undesirable; he became the equivalent of a close relation and therefore could spend time alone with her.
Aisha (Allah be pleased with her) reported that Salim, the freed slave of Abu Hadhaifa, lived with him and his family in their house.
She (i. e. the daughter of Suhail came to Allah's Apostle (may peace be upon him) and said: Salim has attained (puberty) as men attain, and he understands what they understand, and he enters our house freely, I, however, perceive that something (rankles) in the heart of Abu Hudhaifa, whereupon Allah's Apostle (may peace be upon him) said to her: Suckle him and you would become unlawful for him, and (the rankling) which Abu Hudhaifa feels in his heart will disappear.
She returned and said: So I suckled him, and what (was there) in the heart of Abu Hudhaifa disappeared.
Sahih Muslim 8.3425
Examples of an uncle by breast milk:
Aflah asked the permission to visit me but I did not allow him. He said, "Do you veil yourself before me although I am your uncle?"
Aisha said, "How is that?"
Aflah replied, "You were suckled by my brother's wife with my brother's milk."
I asked Allah's Apostle about it, and he said, "Aflah is right, so permit him to visit you."
Narrated Aisha: (the wife of the Prophet) that while Allah's Apostle was with her, she heard a voice of a man asking permission to enter the house of Hafsa.
Aisha added: I said, "O Allah's Apostle! This man is asking permission to enter your house."
The Prophet said, "I think he is so-and-so," naming the foster-uncle of Hafsa.
Aisha said, "If so-and-so," naming her foster uncle, "were living, could he enter upon me?"
The Prophet said, "Yes, for foster suckling relations make all those things unlawful which are unlawful through corresponding birth (blood) relations."
A foster brother could spend time alone with the daughter of the parents who raised him if he had been suckled while in their care before the age of two.
Once the Prophet came to me while a man was in my house. He said, "O 'Aisha! Who is this (man)?"
I replied, "My foster brothers"
He said, "O Aisha! Be sure about your foster brothers, as fostership is only valid if it takes place in the suckling period (before two years of age)."
* Narrated Aisha:
The Prophet and I used to take a bath from a single pot while we were Junub. During the menses, he used to order me to put on an Izar (dress worn below the waist) and used to fondle me. While in Itikaf, he used to bring his head near me and I would wash it while I used to be in my periods (menses).
Junub is an Islamic term meaning "ritually impure (usually) due to sexual intercourse or semen discharge". Wiki