Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) narrates: When the Prophet (peace and blessings be upon him) went into ruku' (bowing), if a cup of water had been placed on his back, the water would not have spilled.
Abdullah bin Ahmad recorded this narration; he says: I found this narration in my father's writing. Its chain contains a narrator who is not named, and there is disagreement about a narrator named Sinan bin Harun.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2739