Musnad Abu Ya'la al-Mawsili 140:18

Abu Khaythama narrated to us, Jarir narrated to us, from al-A'mash, from Yazid al-Raqashi, from Anas ibn Malik, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If a stone, the size of seven pregnant camels with their fat and their offspring, were thrown into Hell, it would fall for seventy years without reaching its bottom."