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)."

حَدَّثَنَا عَلِيُّ بْنُ مُحَمَّدٍ، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ إِدْرِيسَ، عَنِ الْحَسَنِ بْنِ عُبَيْدِ اللَّهِ، عَنْ إِبْرَاهِيمَ بْنِ سُوَيْدٍ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ يَزِيدَ، عَنْ عَبْدِ اللَّهِ، قَالَ قَالَ لِي رَسُولُ اللَّهِ ﷺ " إِذْنُكَ عَلَىَّ أَنْ تَرْفَعَ الْحِجَابَ وَأَنْ تَسْمَعَ سِوَادِي حَتَّى أَنْهَاكَ " .