Sheviis, Chapter 3, Mishnah 7. This Mishnah deals with removing stones from a field during the Sabbatical year, and with the guidelines established so that the act should not appear to be preparing the field for planting.
"Avanim sheziz'ezatan hamachareishah, o shehayu mechusos venisgalu, im yesh bahen shtayim shehen masoi shenayim shenayim - harei elu yinatlu":
"Avanim sheziz'ezatan hamachareishah" - stones that the plow dislodged from their place. The Yerushalmi explains that even stones that have not yet been dislodged, but are destined to be dislodged in the course of the plowing, are included in this ruling.
"O shehayu mechusos venisgalu" - stones that had been covered with earth and became uncovered.
"Im yesh bahen shtayim shehen masoi shenayim shenayim" - two stones that are so heavy that they can only be carried by two people.
"Harei elu yinatlu" - he is permitted to take them. And the Mishnah does not mean those two stones alone; rather, on their account all the stones may be taken, for once stones of such a size are found in the field, taking all of them is permitted.
"Hamesakel es sadeihu":
One who clears stones out of his field in a general way, to the point that he appears to be preparing it for planting - "notel es ha'elyonos, maniach es hanog'os ba'aretz". He must take the upper layer of stones and leave the stones that are touching the ground, in order to make clear that his intention is not for the benefit of the field and its preparation for planting, but rather to use the stones for building and the like.
"Vechen gargar shel tzeroros o gal shel avanim":
The same ruling applies to a large heap of small stones (tzeroros) that he wishes to clear away from the spot, and likewise to a pile of larger stones: "notel es ha'elyonos, maniach es hanog'os ba'aretz" - he takes the upper ones and leaves those that are touching the ground.
"Im yesh tachteihen sela o kash - harei elu yinatlu":
However, if beneath the heap of stones or pebbles there is bedrock, in which planting is impossible, or an entire layer of straw, then it is once again evident that he does not intend to plant specifically in this spot, since there is something there that forms a barrier. Therefore "harei elu yinatlu": he may take all the stones. This is the rule: anything that forms a barrier and prevents the spot from being fit for planting - whether stones, straw, or anything else - permits the removal of all the stones for use in building.
In summary: In this Mishnah we learned that stones which the plow dislodged or which became uncovered, if among them there are two stones that are a load for two people, may all be taken; one who clears stones from his field takes the upper ones and leaves those touching the ground, and the same applies to a heap of pebbles and to a pile of stones - all in order to make clear that his intention is not to prepare the field for planting; and if beneath them there is bedrock or straw, which form a barrier, he may take all of them.