Sunan an-Nasa'i, Book 13, Hadith 95 (In-Book) ⓘ
It was narrated from Sa`d who said: "The Messenger of Allah passed by me when I was supplicating with my fingers and he said: 'Make it one, make it one' and pointed with his forefinger."
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 .
also referenced under: Hadith 1273 (Reference) & Volume 2, Book 13, Hadith 1274 (DarusSalam)