People were instructed to place the right hand over the left forearm during prayer. Abu Hazim said, "I only know that he traced this back to the Prophet ﷺ."
كان الناس يؤمرون أن يضع الرجل اليد اليمنى على ذراعه اليسرى في الصلاة. قال أبو حازم: لا أعلم إلا أنه ينمي ذلك.
Malik narrated it in the chapter on shortening the prayer (47), from Abu Hazim bin Dinar, from Sahl bin Saad, and he narrated it as mentioned. Bukhari narrated it through this same route (740).His saying, "The people were commanded ..." carries the ruling of being raised to the Prophet, because it is understood that the one who commanded them to do this was the Prophet ﷺ.His saying "yanmi dhalika" is with a fatha on the first letter, a sukun on the nun, and a kasra on the mim. The linguists said, "I namaytu the hadith to someone else" means I attributed it and traced it back to him.Maen bin Isa and Ibn Yunus stated this explicitly, as reported from Ismaili and Daraqutni, and Ibn Wahb also added it. All three of them narrated it from Malik with the wording, "yarfau dhalika" (he raises that). It is the convention of the scholars of hadith that when a narrator says "yanmihi," he means that he raises it to the Prophet ﷺ, even if he does not state this explicitly. See Fath al-Bari (2/225).His saying "on his left forearm" necessarily implies placing them on the chest, and this is the sahih and firmly established practice from the Messenger of Allah ﷺ. What has been narrated about placing them above the navel is daif.I recommend here referring to the book Fath al-Ghafur fi Wadh al-Aydi ala al-Sudur by the eminent scholar Sheikh Muhammad Hayat al-Sindi, with my critical edition, third edition, year 1419 AH, in Madinah.