The Book Pertaining to the Remembrance of Allah, Supplication, Repentance and Seeking Forgiveness (Kitab Al-Dhikr)
Muslim :: Book 35 : Hadith 6579
This hadith has been transmitted on the authority of Ibn Abi Laili but with this addition:" Ali said: Ever since I heard this (supplication) from Allah's Apostle (may peace be upon him), I never abandoned it. It was said to him, Not even in the night of Siffin (battle of Siffin)? He sad: Yes, not even in the night of Siffin,"