Quran 18:25 — The Cave (Al-Kahf)

They stayed in their cave three hundred years, increased by nine.*

[18:24] without saying, "GOD willing."* If you forget to do this, you must immediately remember your Lord and say, "May my Lord guide me to do better next time."

[18:26] Say, "GOD is the best knower of how long they stayed there." He knows all secrets in the heavens and the earth. By His grace you can see; by His grace you can hear. There is none beside Him as Lord and Master, and He never permits any partners to share in His kingship.