Sunan Abi Dawud Book 2, Hadith 366
AbuJuhayfah said: Ali said that it is a sunnah to place one hand on the other in prayer below the navel.
AbuUthman an-Nahdi said: When Ibn Mas'ud prayed he placed his left hand on the right. The Prophet (ﷺ) saw him and placed his right hand on his left one.
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.