Majma al-Zawaid — Hadith 2843

Sayyiduna Khufaf bin Ima' bin Rahadah al-Ghifari (may Allah be pleased with him) narrates: When the Prophet (peace and blessings be upon him) sat at the end of his prayer, he would point with his index finger. The polytheists used to say: "He is performing magic with it," whereas the polytheists were lying; it was an affirmation of the Oneness of Allah (tawhid).
Imam Ahmad recorded it as a lengthy narration, and it has already passed in the chapter on the manner of the prayer; Imam al-Tabarani recorded it in al-Mu'jam al-Kabir in the form you see here, and its narrators are thiqat (trustworthy).