משנה על נזיר 7:1

These things defile by overshadowing: a corpse, an olive-sized [portion of flesh] of a corpse, an olive-sized [portion] of nezel, a ladleful of corpse-mold, the spine or the skull, [a] full limb of a corpse, or [a full] limb [severed] from a living person with the appropriate amount of flesh, a quarter [of a kav] of bones from the structural majority or numerical majority, and the structural majority or numerical majority [of the bones] of a corpse even though they do not amount to a quarter [of a kab]; [all these] are unclean. How many [bones] form the numerical majority? One hundred and twenty-five.

[The following likewise defile:]A quarter [of a log] of blood, A quarter [of a log] of mixed blood from one corpse. Rabbi Akiva says: [even] from two corpses. [With regard to] the blood of a child that has completely flowed forth: Rabbi Akiva says: [it defiles] in even the smallest quantity, But the sages say: [there must be] a quarter [of a log]. [With regard to] an olive-sized [portion] of [corpse] worms whether alive or dead: Rabbi Eliezer declares [it] unclean, like the flesh, But the sages declare [it] clean. [With regard to] the ashes of burnt persons: Rabbi Eliezer says: the [minimum] quantity [for defilement is] a quarter [of a kav]. But the sages declare [them to be] clean. A ladleful and [a little] more of grave-dust is unclean. Rabbi Shimon declares [it to be] clean. A ladleful of corpse-mold mixed with water is not [regarded as] joined [into one mass] for [the purposes of] defilement.

One who takes upon himself to become a “chaver” may not sell to an am haaretz either moist or dry [produce], nor may he buy from him moist [produce], nor may he be the guest of an am haaretz, nor may he host an am haaretz as a guest while [the am haaretz] is wearing his own garment. Rabbi Judah says: he may not also raise small animals, nor may make a lot of vows or merriment, nor may he defile himself by contact with the dead. Rather he should be an attendant at the house of study. They said to him: these [requirements] do not come within the general rule [of being a chaver].