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Suhaib narrated: "I passed by Allah's Messenger (S) while he was performing Salat, so I said greeted him with Salam, and he returned it by making signals." He said: "I do not know except that he said: 'He indicated with his finger.'"
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا قُتَيْبَةُ، حَدَّثَنَا اللَّيْثُ بْنُ سَعْدٍ، عَنْ بُكَيْرِ بْنِ عَبْدِ اللَّهِ بْنِ الأَشَجِّ، عَنْ نَابِلٍ، صَاحِبِ الْعَبَاءِ عَنِ ابْنِ عُمَرَ، عَنْ صُهَيْبٍ، قَالَ مَرَرْتُ بِرَسُولِ اللَّهِ صلى الله عليه وسلم وَهُوَ يُصَلِّي فَسَلَّمْتُ عَلَيْهِ فَرَدَّ إِلَىَّ إِشَارَةً . وَقَالَ لاَ أَعْلَمُ إِلاَّ أَنَّهُ قَالَ إِشَارَةً بِإِصْبَعِهِ . قَالَ وَفِي الْبَابِ عَنْ بِلاَلٍ وَأَبِي هُرَيْرَةَ وَأَنَسٍ وَعَائِشَةَ .
sunnah.com