It is narrated from Sayyiduna Khufaf bin Ima' bin Rahadah al-Ghifari (may Allah be pleased with him) that when the Messenger of Allah (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 that he was performing magic with it, but they lied; rather, it was (an expression of) Tawhid (the Oneness of Allah).
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 4161