Musnad Abu Ya'la al-Mawsili 155:193

Abu Hammam narrated to us, Hafs bin Ghiyath narrated to us, from Hisham, from Ibn Sirin, from Abu Hurairah: That the Prophet, peace and blessings of Allah be upon him, saw a man supplicating with both of his fingers together, so he forbade him and said: "With one of them," [meaning] with the right [hand].