"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." (Using translation from Nasāʾī 1186)
عَنْ صُهَيْبٍ صَاحِبَ رَسُولِ اللهِ ﷺ قَالَ «مَرَرْتُ عَلَى رَسُولِ اللهِ ﷺ وَهُوَ يُصَلِّي فَسَلَّمْتُ عَلَيْهِ فَرَدَّ عَلَيَّ إِشَارَةً وَلَا أَعْلَمُ إِلَّا أَنَّهُ قَالَ بِإصْبُعِهِ»