The conditions for the meat an animal slaughtered by to be lawful are as follows:
1. The animal should be slaughtered by someone who is either a Muslim, a Jew, or a Christian.
2. The animal must be killed by a sharp tool that has the ability to provide a clean cut and let the blood flow.
Allah's messenger (peace be upon him) said: "The animal in which the blood flowed and which had Allah's name taken upon it, you may eat." [Sahîh al-Bukhârî
(5503) and Sahîh Muslim
Therefore, regardless of what sharp tool is used, as long as the blood flows after the neck is cut in the proper place, it will be a proper slaughter.
Any further conditions placed upon the method of slaughter itself are not supported by evidence.
2. The Muslim carrying out the slaughter must invoke God's name. According to a number of scholars, this condition applies regardless of whether the person slaughtering the animal is a Muslim, a Christian, or a Jew.
Keeping these conditions in mind, we now turn our attentions to the question of Muslims using automated slaughtering machines in meat production.
It is an acknowledged fact that slaughtering animals by hand is insufficient for our modern society's demand for meat, especially when we speak about the poultry industry. Machines that can slaughter a large number of animals in a short time are needed. There is no objection in Islamic Law against the use of such machines as long as the above-mentioned conditions are met. The specific tools and means of achieving this are an open matter. Any method can be employed.
There are, however, two problems that need to be addressed regarding automated slaughter:
The first of these problems is that it is not possible to mention Allah's name upon each animal when its throat is being cut. The question that needs to be explored here is: Does a single mention of Allah's name suffice for a number of animals?
The answer to this question is that in Islamic Law, a single pronouncement of Allah's name suffices for each group of animals that are killed by a single slaughtering process. This means that if the machines are stopped and restarted, Allah's name will have to be taken again.
This is based on the ruling regarding the killing of multiple game animals with a single shot. We read in the Hanbalî legal treatise Kashshâf al-Qunnâ`
If a hunter kills many animals with a single shot, then the meat of all the animlas is lawful, since the evidence is general.
Likewise, the Hanafî scholar Ibn Nujaym writes in al-Bahr al-Râ'iq
If a hunter shoots an arrow at his prey and manages to kill a number of animals, then his single pronouncement of Allah's name upon firing will suffice for all of them, regardless of how numerous those animals might be… If a man lays two sheep down and manages to slice both of their throats with a single blade, then again one pronouncement of Allah's name will be sufficient for both.
These quotes are sufficient to establish the relevance of this legal principle to the question at hand. There were no automated machines in the past. However, the question of multiple deaths by a single act was experienced with hunting and was addressed by Muslim jurists. They decided that the intention of killing game, regardless of the number of animals killed, in conjunction with the mention of Allah's name is sufficient.
This, in turn, led them to conclude that killing two sheep together with a single knife upon both of their throats would also be valid with a single pronouncement of Allah's name, since the act of slaughter is still being carried out in Allah's name.
This is the same with a machine.
As for instances where there could be doubts as to whether one or more of the conditions for a valid slaughter have been fulfilled, those doubts are to be dismissed.
Ibn al-Qayyim explains in Ahkâm Ahl al-Dhimmah
Whenever there is doubt regarding whether a condition has been fulfilled, and ascertaining the fulfillment of the condition brings about hardship, then the need for certainty regarding the condition is not required. This is the case when a Muslim slaughters an animal – which requires from the Muslim to invoke Allah's name upon slaughtering it. However, we are not required to have knowledge that this condition was carried out.
It is established in the Sunnah that the Prophet (peace be upon him) was asked: "People come to us with meat and w do not know whether or not they had mentioned Allah's name over it."
He told them: "Say Allah's name yourselves, then eat it."
The second issue raised by the machine slaughter of animals is that many machines used for poultry cut the birds' heads off. Also, some machines have a tendency to cut the birds from the back of the neck instead of from the throat.
Neither of these factors make the meat unlawful, since the basic elements of Islamic slaughter are being fulfilled – achieving the flow of blood after opening the throat with a sharp object – with only that more is being cut than is necessary.
Some jurists have said that there is scholarly agreement about the permissibility of eating the meat of animals whose heads were cut off in their entirety during slaughter.
The majority of scholars permit slaughter starting from the back of the neck under the condition that the animal remains alive until its throat is cut. There is no doubt that the extremely quick cuts made by these machines ensures that the animals will still be properly slaughtered, even if the cut starts at the back of the neck.
The ruling that a single pronouncement of Allah's name upon starting up the machine is the official ruling issued by the Islamic Law Council of the Muslim World League in its resolution number 94 issued on 4 July 1997.
It is also in accordance with the fatwa given by Sheikh `Abd al-`Azîz b. Bâz former Grand Muftî of Saudi Arabia.
And Allah knows best.