"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)." (Using translation from Ibn Mājah 139)
قَالَ رَسُولُ اللهِ ﷺ قَدْ أَذِنْتُ لَكَ أَنْ تَرْفَعَ الْحِجَابَ وَتَسْمَعَ سِوَادِي حَتَّى أَنْهَاكَ