Sacrifice - rules, guidelines and modern issues

 In collective sacrifice, is it necessary that all of them should be present at the time of slaughter?

In case of collective sacrifice (like cow, ox, etc.), if all the participants appoint someone as their representative, then it is not necessary that all of them should be present at the time of slaughtering the animal.  It is permissible for the representative to sacrifice the animal on their behalf.  However, if the personally want to sacrifice the animal, it is necessary for each one of them to be present.  In fact, it is better if they participate in the actual slaughter by holding the animal, laying it on its flank, etc. the way the Holy Prophet (Sallallahu alaihi wa sallam) did:

Hadhrat Abu Asad Sulami narrates on the authority of his father

and he in turn from his father that he said:  I was one of the seven

along with the Holy Prophet (Sallallahu alaihi wa sallam).  On the

orders of the Holy Prophet (Sallallahu alaihi wa sallam), we all

contributed 1 Dirham each and bought an animal.  The Holy

Prophet (Sallallahu alaihi wa sallam) said:  Verily, the best sacrifice

in the sight of Allah is the one which is the most expensive and the

most excellent.  Then the Holy Prophet (Sallallahu alaihi wa sallam)

ordered one of us to hold one foreleg, another the other foreleg,

one to hold one hind leg, another the other hind leg and 2 of us to

hold 1 horn each.  Then the last remaining person slaughtered1 the

animal.  All of us said the Takbeer.  Baqiyya (May Allah be well

pleased with him) said:  I asked Hadhrat Hammad (May Allah be

well pleased with him):  Who was that seventh person?  He said:  I

don’t know.  I said:  It was the Holy Prophet (Sallallahu alaihi wa

sallam).  (Kanz Ul Ummal, Hadith No. 12693)