Musnad Ahmad 5:636

Abdullah narrated to us, Abu Bakr ibn Abi Shaybah narrated to me, Abu al-Ahwas narrated to us, from Abu Ishaq, from Asim ibn Damrah, who said: Some of the companions of Ali said to Ali, may Allah be pleased with him: "Will you not tell us about the prayer of the Messenger of Allah, peace and blessings of Allah be upon him, during the day and the voluntary prayers?" Ali said: "You will not be able to endure it." They said to him: "Tell us about it, and we will take from it what we can endure." Then he mentioned the hadith in its entirety.