It is narrated from Sayyiduna Ali (may Allah be pleased with him) that the Sunnah in prayer is that the right hand be placed upon the left hand below the navel.
This Abd al-Rahman bin Ishaq is al-Wasiti al-Qurashi; Ahmad bin Hanbal, Yahya bin Ma'in, al-Bukhari and others criticised him, and Abd al-Rahman bin Ishaq is matruk (abandoned).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2342