Sayyiduna Abu Hurayrah (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "A man's messenger sent to another man is his permission."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأشربة والحد فيها / 17671
It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When one of you is invited and comes along with the messenger, that is his permission."
The Shaykh says that if there is no one whose privacy must be guarded (in the house), then this is permission; otherwise he should come to the door and seek permission again, and this applies after the revelation of the verse of hijab.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأشربة والحد فيها / 17672
Abu Hurayrah (may Allah be pleased with him), in the hadith about the People of the Suffah, says: I came following the Prophet (peace and blessings be upon him) and sought permission, and I was given permission. I found milk in a cup, and he (peace and blessings be upon him) asked: "Where has this come from?" They said: "So-and-so has given it as a gift." He (peace and blessings be upon him) said: "O Abu Hurayrah! Go and call the People of the Suffah." I went, and the People of the Suffah sought permission and entered the house to him (peace and blessings be upon him). Then he narrated the rest of the long hadith.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأشربة والحد فيها / 17673