AbuJuhayfah said: Ali said that it is a sunnah to place one hand on the other in prayer below the navel.
Sunan Abi Dawud Book 2, Hadith 366
← Previous Hadith 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.

Sunan Abi Dawud Book 2, Hadith 365 Next Hadith → 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.

Sunan Abi Dawud Book 2, Hadith 367