Praise be to Allah.
Before answering this question, we must explain some important points about the rulings (ahkaam) concerning breastfeeding (al-radaa’).
1. Breastfeeding is proven in the Qur’aan and Sunnah, and by ijmaa’ (scholarly consensus).
Qur’aan: Allaah says (interpretation of the meaning): “… your foster mothers who gave you suck, your foster milk suckling sisters…” [al-Nisa’ 4:23].
Sunnah: Ibn ‘Abbaas reported that the Prophet (peace and blessings of Allaah be upon him) said: “What is forbidden by radaa’ (suckling or breastfeeding) is the same as what is forbidden by nasab (lineage).”(Agreed upon; al-Bukhaari, Muslim, 1444).
Ijmaa’: The scholars agree that the effect of radaa’ (breastfeeding) prohibits marriage and creates the relationship of mahram, and permits seeing and being alone (with the people to whom one is related through radaa’).
2. For breastfeeding to have the effect of transmitting its benefits from the nursing woman to the child suckled, it must meet certain conditions, which are:
The breastfeeding must happen within the first two years of the child’s life, because Allaah says (interpretation of the meaning): “The mothers shall give suck to their children for two whole years, (that is) for those (parents) who desire to complete the term of suckling…” [al-Baqarah 2:233].
The number of breastfeedings must total the known five feeds, in which the child eats his fill as if eating and drinking. If the child leaves the breast for a reason, such as to take a breath or to switch from one breast to the other, this (i.e., each separate time the child latches on) is not counted as one breastfeeding. This is the opinion of al-Shaafa’i, and the opinion favoured by Ibn al-Qayyim. The definition of rad’ah (one breastfeeding) is when the child sucks at the breast and drinks until the milk enters his stomach, then he leaves the breast of his own accord. The evidence for the number five (number of breastfeedings) is the report from ‘Aa’ishah (may Allaah be pleased with her) who said: “There was in the Qur’aan [an aayah which stipulated that] ten [was the number of] breastfeedings which created the relationship of mahram, then this was abrogated [by another aayah which stipulated] five. The Messenger of Allaah (peace and blessings of Allaah be upon him) died and [the aayah which stipulated five] was still being recited as part of the Qur’aan.”
There are some other reports from the Sahaabah, such as that narrated from Abu ‘Atiyah al-Waadi’i, who said: “A man came to Ibn Mas’ood and said: ‘My wife was with me and her breasts were full of milk (she was engorged). I began to suck it and spit it out. Then I came to Abu Moosa.’ He (Ibn Mas’ood) said, ‘What did you tell him?’ So he (Abu Moosa) told him what he had told him. Then Ibn Mas’ood stood up, took the man’s hand (and said), ‘Do you think this is an infant? Breastfeeding is what produces the growth of flesh and blood.’ Abu Moosa said: ‘Do not ask me anything when this scholar is among you.’”(Reported by ‘Abd al-Razzaaq in al-Musannaf, 7/463, no. 13895).
In al-Muwatta’ (2/603), Maalik reported that Ibn ‘Umar said: “There is no breastfeeding except for the one who is breastfed in infancy; there is no breastfeeding for one who is grown up.” Its isnaad is saheeh.
Maalik also reported in al-Muwatta’ that ‘Abd-Allaah ibn Dinar said: “A man came to ‘Abd-Allaah ibn ‘Umar when I was with him in the court-house, asking him about breastfeeding one who is grown up. ‘Abd-Allaah ibn ‘Umar said: ‘A man came to ‘Umar ibn al-Khattaab and said, “I have a slave-girl with whom I used to have sexual relations, and my wife went to her and gave her her milk, then when I went to her, she said, ‘Stop, by Allaah I have given her my milk.’” ‘Umar said, “Punish her (your wife), and (continue to) go to your slave-girl, for (the ruling on) breastfeeding only applies to breastfeeding of infants.”’” Its isnaad is saheeh.
From this is it clear that drinking one’s wife’s milk has no effect and does not create the relationship of mahram. Ibn Qudaamah said in al-Mughni (9/201): “One of the conditions of breastfeeding creating the relationship of mahram is that it should be within the first two years. This is the opinion of most of the scholars. Something like this was narrated from ‘Umar, ‘Ali, Ibn ‘Umar, Ibn Mas’ood, Ibn ‘Abbaas, Abu Hurayrah and the wives of the Prophet (peace and blessings of Allaah be upon him), apart from ‘Aa’ishah. It was also the opinion of al-Shi’bi, Ibn Shubrumah, al-Oozaa’i, al-Shaafa’i, Ishaaq, Abu Yoosuf, Muhammad, and Abu Thawr, and was narrated in one report from Maalik.
On the basis of the above, drinking one’s wife’s milk has no effect, but it is better to avoid it.
Shaykh Muhammad ibn Saalih al-‘Uthaymeen was asked about this matter, and he replied: Breastfeeding a grown-up has no effect, because the breastfeeding which has an effect (of creating the relationship of mahram) is that which consists of five feedings or more within the first two years, before weaning. On this basis, if it happened that someone breastfed from his wife or drank her milk, he does not become her son. Fataawa Islamiyah, 3/338. And Allaah knows best.