Musnad Ahmad 30:1718

Husayn ibn Muhammad narrated to us, he said: Khalaf ibn Khalifah narrated to us, from Yazid ibn Kaysan, from Abu Hazim, from Abu Hurayrah, who said: We were with the Messenger of Allah (peace and blessings of Allah be upon him) one day, and we heard a loud thud. The Prophet (peace and blessings of Allah be upon him) said: "Do you know what this is?" We said: "Allah and His Messenger know best." He said: "This is a stone that was cast into Hell seventy autumns ago, and it has only now reached its bottom."