The Prophet ﷺ said: Shortening the salutation is sunnah (commendable). The narrator 'Isa said: Ibn al-Mubarak prohibited me from reporting this tradition as a statement of the Prophet ﷺ. Abu Dawud said: I heard Abu 'Umar 'Isa b. Yunus al-Fakhuri al-Ramil saying: When al-Firyabi returned from Mecca, he gave up narrating this tradition as a statement of the Prophet ﷺ. He said: Ahmad b. Hanbal forbade to report this tradition directly from the Prophet ﷺ.  

قَالَ رَسُولُ اللَّهِ ﷺ حَذْفُ السَّلاَمِ سُنَّةٌ قَالَ عِيسَى نَهَانِي ابْنُ الْمُبَارَكِ عَنْ رَفْعِ هَذَا الْحَدِيثِ