Sunan an-Nasa'i 1186

أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا اللَّيْثُ، عَنْ بُكَيْرٍ، عَنْ نَابِلٍ، صَاحِبِ الْعَبَاءِ عَنِ ابْنِ عُمَرَ، عَنْ صُهَيْبٍ، صَاحِبِ رَسُولِ اللَّهِ صلى الله عليه وسلم قَالَ مَرَرْتُ عَلَى رَسُولِ اللَّهِ صلى الله عليه وسلم وَهُوَ يُصَلِّي فَسَلَّمْتُ عَلَيْهِ فَرَدَّ عَلَىَّ إِشَارَةً وَلاَ أَعْلَمُ إِلاَّ أَنَّهُ قَالَ بِإِصْبَعِهِ ‏.‏

It was narrated that Suhaib, the Companion of the Messenger of Allah (ﷺ) said:"I passed by the Messenger of Allah (ﷺ) when he was praying, and greeted him with Salam. He returned my greeting with a gesture, or maybe it was just with his finger

Reference

Sunan an-Nasa'i 1186

In-book

Book 13, Hadith 8