Musannaf Ibn Abi Shayba 3:1564

Waki’ narrated to us, he said: Yazid bin Abi Ziyad narrated to us, from Abu al-Ja’d, from ‘Asim al-Jahdari, from ‘Uqbah bin Zuhayr, from ‘Ali regarding His saying: "So pray to your Lord and sacrifice [al-Kawthar: 2]," he said: "It is placing the right [hand] upon the left in prayer."