It was narrated that 'Abdullah said:
"The Messenger of Allah said to me: 'The sign that you have been permitted to come in is that you raise the curtain and that you hear me speaking quietly, until I forbid you.' (i.e. unless I forbid you)." (PT)
English reference (PT): Vol. 1, Book 1, Hadith 139 (PT)
Arabic reference (PT): Book 1, Hadith 144 (PT)