It is narrated from Ibn Shihab (may Allah have mercy on him) that Abu Ubaydah bin Abdullah bin Zam'ah informed me that his mother, Zaynab bint Abi Salamah (may Allah be pleased with her), informed him that her mother, Umm Salamah (may Allah be pleased with her), the wife of the Prophet (peace and blessings be upon him), used to say: All the wives of the Prophet (peace and blessings be upon him) refused to let anyone enter upon them by virtue of that (kind of) nursing, and they said to Aishah (may Allah be pleased with her): "By Allah, we consider this to be only a concession that the Messenger of Allah (peace and blessings be upon him) granted to Salim specifically. No one shall enter upon us by virtue of this nursing, nor see us." Muslim recorded it in this way in his Sahih.
Imam al-Shafi'i (may Allah have mercy on him) said: Since this was specific to Salim, then what is specific is excluded from the general ruling, and it can only be that the nursing of an adult does not establish prohibition.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرضاع / 15650