The Buddha gives this instruction in the Griha Vinaya (Rules for Householders, Dharmika Sutra, Kshudraka Agama):
Let him not destroy, or cause to be destroyed,
any life at all, or sanction the acts of those who do so.
Let him refrain even from hurting any creature,
both those that are strong and those that tremble in the world.
If we fail to understand the universality of this injunction, the Buddha clarifies (Kshudraka Agama):
Whether they be creatures of the land or air,
whoever harms here any living being,
who has no compassion for all that live,
let such a one be known as depraved.
And in the Anguttara Agama:
I am a friend of the footless,
I am a friend of all bipeds,
I am a friend of those with four feet,
I am a friend of the many-footed.
May all creatures, all breathing things,
all beings one and all, without exception,
experience good fortune only.
May they not fall into any harm.
Should we intend to skirt the First Precept by claiming innocence of the deed if others do the killing for us, He adds (Kshudraka Agama):
One should not kill any living being,
nor cause it to be killed,
nor should one incite any other to kill.
Do never injure any being,
whether strong or weak, in this entire universe!
Source: Let´s not fool ourselves