Musnad Ahmad 5:433

Abdullah narrated to us, he said: I found in my father’s book, he said: I was informed by Sinan bin Harun, Bayan narrated to us, from Abd al-Rahman bin Abi Layla, from Ali bin Abi Talib, he said: "When the Prophet, peace and blessings of Allah be upon him, would bow (in prayer), if a cup of water were placed upon his back, it would not spill."