Halakhah for Niddah 86:56

From the laws of the commandment is that which they, may their memory be blessed, said (Niddah 43b) that the measure for one who touches [it] is like [the size of] a lentil, and for one who [experiences] the emission is with the smallest amount. And it is one, whether he [experiences] the emission accidentally or volitionally, he is impure. And the seed of a young child does not render impure. And red seed does not render impure, but rather [only] white. And any seed that a man does not feel - not at the beginning and not at the end - is not impure. And one who has [improper] thoughts at night, and saw in a dream that he had sexual relations and got up and found his flesh [to be] warm, is impure - even though he did not [knowingly experience] semen. And the rest of its details are elucidated in Tractate Zavim (see Mishneh Torah, Laws of Offerings for Those with Incomplete Atonement 1).