ENOfficial
It was narrated from Sa`d who said: "The Messenger of Allah (ﷺ) passed by me when I was supplicating with my fingers and he said: 'Make it one, make it one' and pointed with his forefinger."
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ الْمُبَارَكِ الْمُخَرِّمِيُّ، قَالَ حَدَّثَنَا أَبُو مُعَاوِيَةَ، قَالَ حَدَّثَنَا الأَعْمَشُ، عَنْ أَبِي صَالِحٍ، عَنْ سَعْدٍ، قَالَ مَرَّ عَلَىَّ رَسُولُ اللَّهِ صلى الله عليه وسلم وَأَنَا أَدْعُو بِأَصَابِعِي فَقَالَ " أَحِّدْ أَحِّدْ " . وَأَشَارَ بِالسَّبَّابَةِ .
sunnah.com