Musnad Ahmad 30:840

Muhammad ibn Abi ‘Adi narrated to us, from Muhammad ibn Ishaq, who said: Muhammad ibn Ibrahim narrated to me, from ‘Isa ibn Talhah, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, a man may speak a word, not intending by it any harm, yet he falls through it seventy autumns into the Fire."