Hadith 756
Narrated Ali ibn Abu Talib: Abu Juhayfah said: Ali said that it is a sunnah to place one hand on the other in prayer below the navel.
Hadith 758
Narrated Abu Hurairah: (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 bin Hanbal say: The narrator Abdur-Rahman bin Ishaq al-Kufi is weak (i. e. not reliable).