Abu al-Qasim Miqsam, the freed slave of Abdullah bin al-Harith bin Nawfal, said: A man from the people of Madinah narrated to me: I prayed in the mosque of Banu Ghifar. Then he mentioned his sitting, and said: I placed my left hand on my left thigh, placed my right hand on my right thigh, and raised my index finger upright. Khufaf bin Ima' bin Rahadah al-Ghifari (may Allah be pleased with him) saw me, and he was a Companion of the Messenger of Allah (peace and blessings be upon him). When I finished my prayer with the salam, he said to me: Why did you raise your finger upright like this? I said: I have seen people doing it like this. He said: You are right. Indeed, when the Messenger of Allah (peace and blessings be upon him) prayed, he would do this, and the polytheists would say that Muhammad (peace and blessings be upon him) does this with his finger in order to cast magic. They lied; the Messenger of Allah (peace and blessings be upon him) did this only to declare with it the Oneness of his Lord, the Blessed and Exalted.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2793