The Prophet ﷺ passed by me while I was supplicating by pointing with two fingers of mine. He said: Point with one finger; point with one finger. He then himself pointed with the forefinger.

مَرَّ عَلَىَّ النَّبِيُّ ﷺ وَأَنَا أَدْعُو بِأُصْبُعَىَّ فَقَالَ أَحِّدْ أَحِّدْ وَأَشَارَ بِالسَّبَّابَةِ