Sunan Abi Dawud Book 2, Hadith 368
(The established way of folding hands is) to hold the hands by the hands in prayer below the navel.
Abu Dawud said: I heard Ahmad b. Hanbal say: The narrator 'Abd al-Rahman b. Ishaq al-Kufi is weak (i.e. not reliable).
I saw Ali (Allah be pleased with him) catching hold of his left hand) by his right hand on the wrist above the navel.
Abu Dawud said: Sa'id b. Jubair narrated the words: "above the navel". Abu Mijlaz reported the words: "below the navel". This has also been narrated by Abu Hurairah. But that is not strong.
The Messenger of Allah (ﷺ) used to place his right hand on his left hand, then he folded them strictly on his chest in prayer.