Sayyiduna Abu Hurairah (may Allah be pleased with him) narrates: The Prophet Muhammad (peace be upon him) saw a man supplicating with two fingers and instructed him to gesture with one finger: which is of the right hand.
(Imam Ibn Hibban, may Allah have mercy on him, says:) There is a subtle indication in this matter, as gesturing with two fingers points to two things, and since those people were close to the time of idol worship and associating partners with Allah, they were commanded to gesture with one finger.