Chapter 39: The Book on the Description of Hellfire
Al-Hasan said: "While he was on the pulpit – the pulpit of Al-Basrah – 'Utbah bin Ghazwain narrated that the Messenger of Allah (s. a. w) said: “Indeed a giant rock can be thrown from the brink of Hell and it will continue to fall into it for seventy years without reaching the bottom of it.”. He said: “' Umar used to say: 'Increase in your remembrance of the Fire, for its heat is extreme, its bottom is distant, and its whips are of Iron.'” (Sahih)
Jami at-Tirmidhi 2575