Sahih Muslim | Book 26 | Chapter 6

Narrated by Ibn Mas'ud
Ibn Mas'ud reported: The Messenger of Allah (may peace be upon him) 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.

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