Regarding the person who asks for permission to enter before he gives the greeting, Abu Hurayra said, "He is not given permission until he has first given the greeting."  

أَنْ يُسَلِّمَ قَالَ لاَ يُؤْذَنُ لَهُ حَتَّى يَبْدَأَ بِالسَّلامِ  

Abu Hurayra said, "When someone comes in and does not say, 'Peace be upon you,' then say, 'No,' until he brings the key which is the greeting (salam)."  

سَمِعْتُ أَبَا هُرَيْرَةَ يَقُولُ إِذَا دَخَلَ وَلَمْ يَقُلِ السَّلاَمُ عَلَيْكُمْ فَقُلْ لاَ حَتَّى يَأْتِيَ بِالْمِفْتَاحِ السَّلامِ