This is the first verse of the Icelandic wisdom poem Havamal:
1. At every door-way,
ere one enters,
one should spy round,
one should pry round
for uncertain is the witting
that there be no foeman sitting,
within, before one on the floor.
(As translated by Olive Bray - I believe this text to be out of copyright and it's available all over the net). If you're not familiar with the Havamal it is 'The Words of the High One' (i.e Odin), a poem from the collection known as the Poetic Edda. The remaining manuscript dates from the 13th Century, although individual verses are certainly much older. It is the core of the modern Heathen moral outlook although, as with all ancient texts, translations and interpretations differ widly. To me it's a bunch of common sense, whether it really originated with Odin or with Sven the Scriptorium Keeper I don't know and don't really think it matters much.
This verse is one that some internet wanderers could benefit from methinks, having a tendency to wander in to a site and start spouting their opinions without checking whether they're really welcome, whether there are foemen within. The website becomes a virtual hall, often with its own community, kindred, of people with a distinct group attitude. Obviously, these days it's less of an issue in real life than it would have been in the Viking Age, but still, making sure you're on friendly turf is wise in real life too. This verse begins the Havamal and also begins a section of verses concentrating on hospitality, being a good guest and not making an arse of yourself in polite company.
I find it amusing to consider what would have happened to the ancient equivalent of the internet troll back in 10th Century Iceland. They'd have been a short lived species I think...