مَا مِنْ أَحَدٍ يُسَلِّمُ عَلَيَّ إِلَّا رَدَّ اللَّهُ عَلَيَّ رُوحِي حَتَّى أَرُدَّ عَلَيْهِ السَّلَامَ
No one sends greetings of peace to me without Allah returning my spirit to me so that I respond to the greeting. (Narrated by Abu Hurayrah; Sunan Abi Dawud)
Responding to greetings is cited as evidence for the Prophet’s distinctive life in barzakh. Its mode is not required to resemble ordinary conversation in every respect.