Musnad Ahmad 26:377

Aswad narrated to us, Isra'il informed us, from Abu Ishaq, from Ibn al-Aswad, from Alqamah and al-Aswad: That they were with Ibn Mas'ud, and the time for prayer arrived. Alqamah and al-Aswad stepped back, so Ibn Mas'ud took their hands and placed one of them to his right and the other to his left. Then they bowed and placed their hands upon their knees. He struck their hands, then he joined his hands together and interlaced them, and placed them between his thighs. And he said: "I saw the Prophet, may Allah bless him and grant him peace, do this."