كتاب الطلاق
18  
The Book of Divorce
(4)
Chapter: Giving one's wife the choice does not count as a divorce, unless it is intended as such
(4)
باب بَيَانِ أَنَّ تَخْيِيرَ امْرَأَتِهِ لاَ يَكُونُ طَلاَقًا إِلاَّ بِالنِّيَّةِ ‏‏
Sahih Muslim 1477 b

Masruq reported:

I do not mind if I give option to my wife (to get divorce) once, hundred times, or thousand times after (knowing it) that she has chosen me (and would never seek divorce). I asked 'A'isha (Allah be pleased with her) (about it) and she said: Allah's Messenger (ﷺ) gave us the option, but did it imply divorce? (It was in fact not a divorce; it is effective when women actually avail themselves of it.)
وَحَدَّثَنَاهُ أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا عَلِيُّ بْنُ مُسْهِرٍ، عَنْ إِسْمَاعِيلَ بْنِ أَبِي، خَالِدٍ عَنِ الشَّعْبِيِّ، عَنْ مَسْرُوقٍ، قَالَ مَا أُبَالِي خَيَّرْتُ امْرَأَتِي وَاحِدَةً أَوْ مِائَةً أَوْ أَلْفًا بَعْدَ أَنْ تَخْتَارَنِي وَلَقَدْ سَأَلْتُ عَائِشَةَ فَقَالَتْ قَدْ خَيَّرَنَا رَسُولُ اللَّهِ صلى الله عليه وسلم أَفَكَانَ طَلاَقًا.
Reference : Sahih Muslim 1477b
In-book reference : Book 18, Hadith 34
USC-MSA web (English) reference : Book 9, Hadith 3501
  (deprecated numbering scheme)