Abu al-Abbas said: "And he (peace and blessings be upon him) forbade arriving unexpectedly at a meal (without an invitation); and Sayyiduna Abu al-Darda' (may Allah be pleased with him) arrived unexpectedly at his (peace and blessings be upon him) meal, and he (peace and blessings be upon him) ordered him to eat; and this was particular to him (peace and blessings be upon him)."
The Shaykh (Imam al-Bayhaqi) (may Allah have mercy on him) said: "I do not know the hadith forbidding arriving unexpectedly at a meal in this form through any chain the like of which is established; what I know is among the reports that carry part of its meaning."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / Q13412
Abdullah bin Umar (may Allah be pleased with them both) said that the Prophet (peace and blessings be upon him) said: "Whoever is invited and does not accept has disobeyed Allah and the Messenger, and whoever enters without an invitation enters as a thief and leaves as a raider."
This was said concerning a man who goes in to another, knowing that he is eating, in order to eat with him. And a hadith has been narrated that negates the special exemption that Abu al-Abbas imagined regarding the meal of the Prophet (peace and blessings be upon him) in the story of Abu al-Darda'.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13412
Jabir (may Allah be pleased with him) said: One day the Prophet (peace and blessings be upon him) went towards Shi'b al-Jabal (the mountain pass); he (peace and blessings be upon him) relieved himself, and there were dates lying on our iron or leather shield. We invited him (peace and blessings be upon him). He (peace and blessings be upon him) ate with us and did not touch water.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13413
It is also narrated from Sayyiduna Jabir (may Allah be pleased with him) that they were eating dates on a shield. He said: "The Messenger of Allah (peace and blessings be upon him) passed by us, having come from relieving himself, so we said: 'Come!' So he (peace and blessings be upon him) sat down and ate the dates with us, and did not touch water."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13414
Sayyiduna al-Ash'ath bin Qays (may Allah be pleased with him) came to Sayyiduna Abdullah (may Allah be pleased with him) on the day of Ashura while he was eating. He said: O Abu Muhammad! Come near and eat. He said: I am fasting. He said: We too used to fast (on this day), then it was abandoned.
There are many reports on this, and all of them negate the special exemption; and Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13415