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. (Using translation from Abū Dāʾūd 1499)
مَرَّ عَلَيَّ النَّبِيُّ ﷺ وَأَنَا أَدْعُو بِأَصَابِعِي فَقَالَ «أَحَدٌ أَحَدٌ» وَأَشَارَ بِالسَّبَّابَةِ