Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates that the Prophet of Allah (peace and blessings be upon him) said: "Your permission to come to me is that you lift the curtain and hear my private talk, until I forbid you." Al-Hasan said: "Al-sawad" means private talk.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8417