Allah, glorified be His praise, says: "And it is not for any human being that Allah should speak to him except by revelation, or from behind a veil, or that He sends a messenger (an angel) who reveals by His permission what He wills." [Surah al-Shura: 51]
And Allah Almighty said concerning the hypocrites, (on the tongue) of the believers: "Say: Make no excuses; we will never believe you. Allah has already informed us of your news." [Surah al-Tawbah: 94]
And Allah Almighty informed them of their (the hypocrites') affairs only through the revelation which Jibril (peace be upon him) brought down to the Prophet (peace and blessings be upon him), and the Prophet (peace and blessings be upon him) informed them (of their affairs) through the revelation that came to him.
Imam al-Shafi'i (may Allah have mercy on him) said: "Whoever says that he does not break his oath (by writing a letter or sending a messenger) says that the speech of human beings does not resemble the speech of Allah; the speech of human beings is face to face. Do you not see that if a man cuts off relations with another man, while cutting off relations for more than three nights is forbidden to him, and then he writes to him or sends a messenger to him while he is able to speak to him, this does not take him out of the cutting off of relations for which he is sinful?"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / Q20027
Sayyiduna Abu Ayyub al-Ansari (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "It is not lawful for a Muslim to cut off relations with his Muslim brother for more than three days, such that they meet yet each holds back; and the better of the two is the one who begins with the greeting of salam."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 20027
Muhammad bin Hilal narrates from his father: I heard Sayyiduna Abu Hurayrah (may Allah be pleased with him) say that the Prophet (peace and blessings be upon him) said: "It is not lawful for a believer to forsake his believing brother for more than three days; but after three days have passed, they should greet each other with salam. If he replies, both of them will share in the reward; and if he does not reply, then this one (who did not reply) will be the wrongdoer, and the other will be free of (the sin of) cutting off relations."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 20028