Sahih Muslim
The Book Pertaining to Paradise, Its Description, Its Bounties and Its Intimates (Kitab Al-Jannat wa Sifat Naimiha wa Ahliha)
Book Number: 040 / Hadith Number: 6826
Anas reported Allah's Apostle (may peace be upon him) 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.