What is the standard for determining the quality of livestock taken for Zakah when the herd is divided into three quality tiers?

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When the Zakah collector arrives, he should divide the sheep into three segments: one-third excellent, one-third average, and one-third poor. The collector must take from the average portion. This ruling is reported from 'Umar (RA) and is the position of our Imam (Ahmad ibn Hanbal). Evidence supports this, as a narration states that the Prophet's (PBUH) messengers refused to take a pregnant sheep (Shafi'), which is one carrying milk (Mukhad) and whose offspring are near delivery, indicating that specific superior or vulnerable categories are excluded.