Narrated `Aisha:that the Prophet (ﷺ) entered upon her while a man was sitting with her. Signs of answer seemed to appear on his face as if he disliked that. She said, "Here is my (foster) brother." He said, "Be sure as to who is your foster brother, for foster suckling relationship is established only when milk is the only food of the child."
حَدَّثَنَا أَبُو الْوَلِيدِ، حَدَّثَنَا شُعْبَةُ، عَنِ الأَشْعَثِ، عَنْ أَبِيهِ، عَنْ مَسْرُوقٍ، عَنْ عَائِشَةَ ـ رضى الله عنها ـ أَنَّ النَّبِيَّ صلى الله عليه وسلم دَخَلَ عَلَيْهَا وَعِنْدَهَا رَجُلٌ، فَكَأَنَّهُ تَغَيَّرَ وَجْهُهُ، كَأَنَّهُ كَرِهَ ذَلِكَ فَقَالَتْ إِنَّهُ أَخِي. فَقَالَ " انْظُرْنَ مَا إِخْوَانُكُنَّ، فَإِنَّمَا الرَّضَاعَةُ مِنَ الْمَجَاعَةِ ".
Bukhari
Wedlock, Marriage (Nikaah) • Hadith 40