This is from De Bello Gallico by Caesar...........
.... And while the attention of our men is engaged in that matter, in another part Adcantuannus, who held the chief command, with 600 devoted followers whom they call soldurii (the conditions of whose association are these, - that they enjoy all the conveniences of life with those to whose friendship they have devoted themselves: if any thing calamitous happen to them, either they endure the same destiny together with them, or commit suicide: nor hitherto, in the, memory of men, has there been found any one who, upon his being slain to whose friendship he had devoted himself, refused to die); Adcantuannus, endeavoring to make a sally with these, when our soldiers had rushed together to arms, upon a shout being raised at that part of the, fortification, and a fierce battle had been fought there, was driven back into the town, yet he obtained from Crassus [the indulgence] that he should enjoy the same terms of surrender [as the other inhabitants].
Book III Chapt. 22