It is narrated from Abu al-Qasim Miqsam (may Allah have mercy on him): A man from the people of Madinah narrated to me: I prayed beside Khufaf bin Ima' bin Rahadah (may Allah be pleased with him). He saw me pointing with my finger in the prayer, and he said: O my nephew, why do you do this? I said: I have seen the righteous and the jurists doing it like this. He said: You have spoken rightly; I saw the Messenger of Allah (peace and blessings be upon him): when he sat for the tashahhud in the prayer, he would point with his finger. The polytheists would say: He is only casting magic on us. Whereas the Messenger of Allah (peace and blessings be upon him) intended tawhid (the Oneness of Allah) by it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2792