The Messenger of Allah ﷺ said, "There will remain in Paradise whatever Allah wills to remain, and Allah will create for it whatever creation He wills."
حدثنا سليمان بن حرب حدثنا حماد عن ثابت
قال رسول الله ﷺ يبقى من الجنة ما شاء الله أن يبقى فينشئ الله لها خلقا ما شاء
Its isnad is sahih. Bukhari related it in 13/369 no. 7384 in Fath, in the Book of Tawhid, as did Muslim in the Book of Paradise 4/2188 no. 2148, and Ibn Hibban 16/485 no. 7448 in al-Ihsan.The meaning of the hadith is not that the people of Paradise will diminish. Rather, its meaning is that because of the continuous expansion of Paradise, Allah creates new beings who will live in that expansion, and Allah has many affairs concerning His creation.