Musnad Ahmad 5:133

Hujayn narrated to us, Isra'il narrated to us, from 'Abd al-A'la, from Abu 'Abd al-Rahman, from 'Ali ibn Abi Talib, from the Prophet (peace and blessings of Allah be upon him), who said: "Whoever lies about me in his dream, he will be tasked with tying a knot in a barley grain on the Day of Resurrection."