The Messenger of Allah ﷺ said to me: The sign that you have been permitted to come in is that the curtain is raised or that you hear me speaking quietly until I forbid you.  

لِي رَسُولُ اللَّهِ ﷺ إِذْنُكَ عَلَىَّ أَنْ يُرْفَعَ الْحِجَابُ وَأَنْ تَسْتَمِعَ سِوَادِي حَتَّى أَنْهَاكَ  

This hadith has been narrated on the authority of 'Ubaidullah with the same chain of transmitters.  

عَنِ الْحَسَنِ بْنِ عُبَيْدِ اللَّهِ بِهَذَا الإِسْنَادِ مِثْلَهُ