the Fatwa Department Research Committee - chaired by Sheikh `Abd al-Wahhâb al-Turayrî
The state of janâbah is a state or ritual impurity on account of sexual activity. When a Muslim is in the state of janâbah, he or she is prohibited from engaging in certain acts of worship.
There is nothing in Islamic law to prohibit a woman from breastfeeding her baby while she is in a state of janâbah.
In fact, such a restriction would go against the very wisdom of Islamic Law that seeks ease and facilitation and that seeks to save people from injury and harm. Such a ruling would bring great difficulty to the mother and could lead to serious harm for the baby who is in need of sustenance.
A Muslim should never feel personally impure because she is in a state of janâbah. The state of janâbah is a state of ritual impurity (hadath) not a state of physical impurity (khabath). It is an abstract state wherein a person cannot perform certain acts of worship.
The Prophet (peace be upon him) made this clear to us.
Abû Hurayrah relates to us that he saw the Prophet (peace be upon him) while he was walking down a road in Madînah. Abû Hurayrah hid himself out of sight of the Prophet (peace be upon him).
Then he went off, bathed himself, and went to the Prophet (peace be upon him).
The Prophet (peace be upon him) asked him: “Where have you been, Abû Hurayrah?”
He said: “I was in a state of major ritual impurity (janâbah) and I disliked that I should sit with you while in such a state.”
The Prophet (peace be upon him) said: “Glory to Allah! A Muslim never becomes impure.” [Sahîh al-Bukhârî (283) and Sahîh Muslim (371)]
This shows us that the state of janâbah does not make a person physically impure. It is merely a state of ritual impurity. It certainly does not make a woman’s breast milk impure.
A person in a state of janâbah can do everything except engage in certain acts of worship. He or she can engage in all mundane activities, including shopping, cooking, eating, and taking care of children.
And Allah knows best.
Source: Islam Today