Musnad Ahmad 1013:52

'Abd al-Rahman ibn Mahdi narrated to us, from Malik, from Abu Hazim, from Sahl ibn Sa'd, who said: "The people were commanded to place the right [hand] upon the left in prayer." Abu Hazim said: "I do not know [it] except that he attributes that [to the Prophet]." Abu 'Abd al-Rahman [Ahmad ibn Hanbal] said: "He attributes it [means] he raises it to the Prophet, may Allah bless him and grant him peace."