Language:
كتاب البيوع
34  
Sales and Trade
(43)
Chapter: If the time for the option is not fixed, will the deal be considered as legal?
(43)
باب إِذَا لَمْ يُوَقِّتْ فِي الْخِيَارِ، هَلْ يَجُوزُ الْبَيْعُ
Sahih al-Bukhari 2109

Narrated Ibn `Umar:

Allah's Messenger (ﷺ) said, "The seller and the buyer have the option of canceling or confirming the deal unless they separate, or one of them says to the other, 'Choose (i.e. decide to cancel or confirm the bargain now)." Perhaps he said, 'Or if it is an optional sale.' " Ibn `Umar, Shuraih, Ash-Shu`bi, Tawus, Ata, and Ibn Abu Mulaika agree upon this judgment.

حَدَّثَنَا أَبُو النُّعْمَانِ، حَدَّثَنَا حَمَّادُ بْنُ زَيْدٍ، حَدَّثَنَا أَيُّوبُ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ ـ رضى الله عنهما ـ قَالَ قَالَ النَّبِيُّ صلى الله عليه وسلم ‏ "‏ الْبَيِّعَانِ بِالْخِيَارِ مَا لَمْ يَتَفَرَّقَا، أَوْ يَقُولُ أَحَدُهُمَا لِصَاحِبِهِ اخْتَرْ ‏"‏‏.‏ وَرُبَّمَا قَالَ أَوْ يَكُونُ بَيْعَ خِيَارٍ‏.‏
Reference : Sahih al-Bukhari 2109
In-book reference : Book 34, Hadith 62
USC-MSA web (English) reference : Vol. 3, Book 34, Hadith 322
  (deprecated numbering scheme)