كتاب الجنة وصفة نعيمها وأهلها
53
The Book of Paradise, its Description, its Bounties and its Inhabitants
(13)
Chapter: The Arrogant Will Enter The Fire, And The Humble Will Enter Paradise
(13)
باب النَّارُ يَدْخُلُهَا الْجَبَّارُونَ وَالْجَنَّةُ يَدْخُلُهَا الضُّعَفَاءُ

Anas reported Allah's Apostle (ﷺ) as saying:

There would be left some space in Paradise as Allah would like that to be left. Then Allah would create another creation as He would like.
حَدَّثَنِي زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا عَفَّانُ، حَدَّثَنَا حَمَّادٌ، - يَعْنِي ابْنَ سَلَمَةَ - أَخْبَرَنَا ثَابِتٌ، قَالَ سَمِعْتُ أَنسًا، يَقُولُ عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏ "‏ يَبْقَى مِنَ الْجَنَّةِ مَا شَاءَ اللَّهُ أَنْ يَبْقَى ثُمَّ يُنْشِئُ اللَّهُ تَعَالَى لَهَا خَلْقًا مِمَّا يَشَاءُ ‏"‏ ‏.‏
Reference : Sahih Muslim 2848 d
In-book reference : Book 53, Hadith 49
USC-MSA web (English) reference : Book 40, Hadith 6826
  (deprecated numbering scheme)