Musannaf Ibn Abi Shayba 3:6049

Abu Bakr narrated to us, he said: Hafs bin Ghiyath narrated to us, from al-A’mash, from Abu Salih, from Abu Hurayrah, who said: The Prophet, peace and blessings of Allah be upon him, saw Sa’d while he was supplicating with both of his fingers. So he forbade him and said: "With one finger, with the right one."