Abu Hurayra reported that he heard the Prophet, may Allah bless him and grant him peace, say, "It is not lawful for a Muslim man to refuse to speak to a believer for more than three days. When three days have passed, he should meet him and greet him. If he returns the greeting, they share in the reward. If he does not return the greeting, the one who gives the greeting is innocent of having severed relations."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا إِسْمَاعِيلُ بْنُ أَبِي أُوَيْسٍ قَالَ: حَدَّثَنِي مُحَمَّدُ بْنُ هِلاَلِ بْنِ أَبِي هِلاَلٍ مَوْلَى ابْنِ كَعْبٍ الْمَذْحِجِيِّ، عَنْ أَبِيهِ، أَنَّهُ سَمِعَ أَبَا هُرَيْرَةَ قَالَ: سَمِعْتُ النَّبِيَّ صلى الله عليه وسلم يَقُولُ: لاَ يَحِلُّ لِرَجُلٍ أَنْ يَهْجُرَ مُؤْمِنًا فَوْقَ ثَلاَثَةِ أَيَّامٍ، فَإِذَا مَرَّتْ ثَلاَثَةُ أَيَّامٍ فَلْيَلْقَهُ فَلْيُسَلِّمْ عَلَيْهِ، فَإِنْ رَدَّ عَلَيْهِ السَّلاَمَ فَقَدِ اشْتَرَكَا فِي الأَجْرِ، وَإِنْ لَمْ يَرُدَّ عَلَيْهِ فَقَدْ بَرِئ الْمُسْلِمُ مِنَ الْهِجْرَةِ.
sunnah.com