Once the Messenger of Allah (may Allah's peace and blessings be upon him) entered upon me while a man was in my house. He said: O ‘Ā'ishah, who is this (man)? I replied: He is my brother from breastfeeding. He said: O ‘Ā'ishah, be careful who you (women) count as your brothers from breastfeeding, for the breastfeeding (that makes marriage prohibited) is only that which satisfies hunger

The Prophet (may Allah's peace and blessings be upon him) entered upon ‘Ā'ishah and found her brother from breastfeeding, whom he did not know, with her. So, his facial expression changed, in dislike of that situation, and out of jealously for his female relatives. ‘Ā'ishah knew the reason why his facial expression had changed, so she told him that the man was her brother from breastfeeding. Thereupon, the Prophet (may Allah's peace and blessings be upon him) said: O ‘Ā'ishah, make sure of those whom you consider your brothers from breastfeeding, because there are some types of breastfeeding that do not make marriage prohibited. It is necessary that the breastfeeding nourishes the child and grows it muscles and strengthens its bones. This only occurs during the first two years of the child's life, when the he is in need of breast milk and feeds on nothing else. At this time, the child becomes part of the breastfeeding mother, and is considered as one of her own children. Thus, the prohibition of marriage is established, i.e. he becomes a Mahram for her and her family. This means that she must not put on her Hijab in his presence, he can sit alone with her, and he can be her Mahram on traveling. This includes the breastfeeding mother, her husband, their children, their siblings, and their mothers and fathers.