Sunan Abi Dawud Book 2, Hadith 367
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.
AbuJuhayfah said: Ali said that it is a sunnah to place one hand on the other in prayer below the navel.
(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).