Jami` at-Tirmidhi 2575

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)