كتاب الرضاع
17  
The Book of Suckling
(14)
Chapter: It is permissible for a wife to give her turn to a co-wife
(14)
باب جَوَازِ هِبَتِهَا نَوْبَتَهَا لِضَرَّتِهَا ‏‏
Sahih Muslim 1463 a

'A'isha (Allah be pleased with her) reported:

Never did I find any woman more loving to me than Sauda bint Zam'a. I wished I could be exactly like her who was passionate. As she became old, she had made over her day (which she had to spend) with Allah's Messenger (ﷺ) to 'A'isha. She said: I have made over my day with you to 'A'isha. So Allah's Messenger (ﷺ) allotted two days to 'A'isha, her own day (when it was her turn) and that of Sauda.
حَدَّثَنَا زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا جَرِيرٌ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ، قَالَتْ مَا رَأَيْتُ امْرَأَةً أَحَبَّ إِلَىَّ أَنْ أَكُونَ فِي مِسْلاَخِهَا مِنْ سَوْدَةَ بِنْتِ زَمْعَةَ مِنِ امْرَأَةٍ فِيهَا حِدَّةٌ قَالَتْ فَلَمَّا كَبِرَتْ جَعَلَتْ يَوْمَهَا مِنْ رَسُولِ اللَّهِ صلى الله عليه وسلم لِعَائِشَةَ قَالَتْ يَا رَسُولَ اللَّهِ قَدْ جَعَلْتُ يَوْمِي مِنْكَ لِعَائِشَةَ ‏.‏ فَكَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَقْسِمُ لِعَائِشَةَ يَوْمَيْنِ يَوْمَهَا وَيَوْمَ سَوْدَةَ ‏.‏
Reference : Sahih Muslim 1463a
In-book reference : Book 17, Hadith 62
USC-MSA web (English) reference : Book 8, Hadith 3451
  (deprecated numbering scheme)