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וראשית גז צאנך תתן לו (דברים יח:ד)י

מסכת חולין פרק יא משנה א
ראשית הגז The first of the shearing (when shearing sheep, the first of the shearing must be given to a kohen),
נוהג בארץ ובחוצה לארץ is practiced in the Land [of Israel] and outside the Land,
בפני הבית ושלא בפני הבית before the Temple [while it stood] and not before the Temple
בחלין by unsanctified animals
אבל לא במקדשין but not by sanctified animals.
חמר בזרוע ובלחיים ובקבה The foreleg, jaw and maw (the matanos) are more stringent
מראשית הגז than the first of the shearing
שהזרוע והלחיים והקיבה as the foreleg, jaw and maw
נוהגים בבקר ובצאן are practiced by cattle and by sheep
במרבה ובמועט by many [animals] and by few,
וראשית הגז and the first of the shearing
אינו נוהג אלא ברחלות is not practiced but by sheep
ואינו נוהג אלא במרבה and is not practiced except by [groups of] many [sheep].

פרק יא משנה ב

וכמה הוא מרובה And how many are many [such that the obligation of the first of the shearing will apply]?
בית שמאי אומרים Beis Shammai say
שתי רחלות two sheep,
שנאמר as it says
יחיה איש עגלת בקר "A man will support a calf
ושתי צאן and two sheep," (showing that the term "צאן" is used for as few as two sheep, corresponding to the term "צאנך" by the mitzvah of the first of the shearing.)
י(ישעיהו ז:כא)י  
ובית הלל אומרים חמש and Beis Hillel say five,
שנאמר as it says
וחמש צאן עשויות "and five sheep prepared" (the term "עשויות" is interpreted to mean that "they create"
י(שמואל א כה:יח)י  
רבי דוסא בן הרכינס אומר Rabbi Dosa ben Horkinos says
חמש רחלות five sheep
גוזזות מנה מנה ופרס shorn each a maneh and a half (This is roughly 1.6 pounds of wool per sheep, or 8 lbs in total.  He holds that less than that amount of wool from a sheep is not considered a shearing.)
חיבות בראשית הגז are obligated in the first of the shearing
וחכמים אומרים and the Sages say
חמש רחלות five sheep
גוזזות כל שהן that are shorn in any amount.
וכמה נותנין לו And how much is given to him [the kohen]?

משקל חמש סלעים ביהודה

The weight of five selaim in Judea
שהן עשר סלעים בגליל which are [the equivalent of] ten selaim in Galilee,
מלובן ולא צואי bleached and not soiled (it need not actually be bleached, but it must be sufficient to meet this weight requirement after being bleached),
כדי לעשות ממנו בגד קטן such that a small garment could be made from it,
שנאמר as it says
תתן לו "you shall give to him"
י(דברים יח:ד)י  
שיהא בו כדי מתנה such that there is enough of it to be a [useful] gift.
לא הספיק לתנו לו If he did not get around to giving it to him

עד שצבעו

until after he dyed it,

he is exempt.  (By changing it so, if accompanied by intent to steal this wool from the kohanim, it is an act of conversion such that the wool is now his property.  Although normally one who converts someone else's property to their own would be forced to pay for it, here no one kohen who can lay claim to this wool, and so he cannot be forced to pay).

לבנו ולא צבעו If he bleached it but did not dye it
חיב he is obligated.
הלוקח גז צאנו של נכרי One who buys the shearing of sheep of a gentile
פטור מראשית הגז is exempt from the first of the shearing.
הלוקח גז צאנו של חברו One who buys the shearing of sheep of his colleague,
אם שיר המוכר if the seller left some over (i.e. the seller did not sell all of the sheep's wool)
המוכר חיב the seller is obligated (as we assume that he did not intend to sell the kohen's property)
ואם לאו and if not
הלוקח חיב the buyer is obligated.
היו לו שני מינים If he [the seller] had two kinds [of sheep],
שחופות ולבנות dark ones and light ones (light ones being superior),
מכר לו שחופות and he sold him the dark ones
אבל לא לבנות but not the light ones,
זכרים אבל לא נקבות [or he sold him] the males but not the females (females having softer and superior wool),
זה נותן לעצמו this one must give for himself
וזה נותן לעצמו and this one must give for himself.