'Abdullah ibn 'Umar said, "When someone enters a house which is not lived in, he should say, 'Peace be upon the righteous slaves of Allah.'"  

أَنَّ عَبْدَ اللهِ بْنَ عُمَرَ قَالَ إِذَا دَخَلَ الْبَيْتَ غَيْرَ الْمَسْكُونِ فَلْيَقُلِ السَّلاَمُ عَلَيْنَا وَعَلَى عِبَادِ اللهِ الصَّالِحِينَ  

Ibn 'Abbas said, "The verse, 'Do not enter houses other than your own until you have asked permission and greeted their inhabitants' (24:27) has an exception made to it when Allah says, 'There is nothing wrong in your entering houses where no one lives where there is some service for you. Allah knows what you divulge and what you conceal.'"  

أَنْ تَدْخُلُوا بُيُوتًا غَيْرَ مَسْكُونَةٍ فِيهَا مَتَاعٌ لَكُمْ وَاللهُ يَعْلَمُ مَا تُبْدُونَ وَمَا تَكْتُمُونَ}