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
← Previous Hadith I heard Ibn al-Zubair say: Setting the feet right and placing one hand on the other is a sunnah.

Sunan Abi Dawud Book 2, Hadith 364 Next Hadith → 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