It is narrated from Abu Tufail that a voice from the unseen was heard by Rasulullah (sallallahu ‘alaihi wasallam) saying, “O Muhammed (sallallahu ‘alaihi wasallam)! Cover your satr!” This was the first voice from the unseen that Rasulullah (sallallahu ‘alaihi wasallam) heard.
According to the narration of Hazrat ‘Abdullah bin ‘Abbaas (radhiyallahu ‘anhuma), Abu Taalib asked Rasulullah (sallallahu ‘alaihi wasallam) what transpired. He replied, “A person dressed in white clothing appeared and said to me, ‘O Muhammed (sallallahu ‘alaihi wasallam)! Conceal your satr!’”
(Extracted from Seeratul Mustafa 1/119)