Sunan Abi Dawud Book 2, Hadith 365

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.

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
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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