משנה על ברכות 3:1

If one finds a corpse unexpectedly lying in its natural position, he may remove it along with the [blood-] saturated earth around it. If he finds two, he may remove them along with the [blood-] saturated earth around it. If he finds three, if there is a space of from four to eight cubits between the first and the last, that is, the space of a bier and its bearers, then it must be accounted a graveyard. He must search [the ground] for twenty cubits from that point. If he found [another corpse] at the end of those twenty cubits, he must search for a further twenty cubits from that place, since there are already grounds for belief [that this is a graveyard], in spite of the fact that if he had found this [lone grave] in the first case, he could have removed it with the [blood-] saturated earth around it.