Musnad Abu Ya'la al-Mawsili 6:105

Abu Kurayb narrated to us, Mu'awiyah narrated to us, al-A'mash narrated to us, from Abu Salih, from Sa'd ibn Abi Waqqas, who said: The Prophet, peace and blessings of Allah be upon him, passed by while I was supplicating with my finger. The Messenger of Allah, peace and blessings of Allah be upon him, said: "One, One," and he gestured with his index finger.