Human beings are like parts of a body,
created from the same essence.
When one part is hurt and in pain,
the others cannot remain in peace and be quiet.
If the misery of others leaves you indifferent
and with no feelings of sorrow,
You cannot be called a human being.

Sa'adi (1184-1283/1291?) Persian poet [a.k.a. Sa'di, Moslih Eddin Sa'adi, Mushrif-ud-Din Abdullah, Muslih-ud-Din Mushrif ibn Abdullah, Mosleh al-Din Saadi Shirazi, Shaikh Mosslehedin Saadi Shirazi]
Poem on Humanity

Alternate translations:

Posted in the entrance to the Hall of Nations of the UN building, New York:

Of one Essence is the human race,
Thusly has Creation put the Base;
One Limb impacted is sufficient,
For all Others to feel the Mace.

Alt trans (M. Aryanpour):

Human beings are members of a whole,
In creation of one essence and soul.
If one member is afflicted with pain,
Other members uneasy will remain.
If you have no sympathy for human pain,
The name of human you cannot retain.

Alternate:

The sons of Adam are limbs of each other,
Having been created of one essence.
When the calamity of time affects one limb
The other limbs cannot remain at rest.
If thou hast no sympathy for the troubles of others
Thou art unworthy to be called by the name of a human.

Alternate:

All human beings are in truth akin;
All in creation share one origin.
When fate allots a member pangs and pains,
No ease for other members then remains.
If, unperturbed, another's grief canst scan,
Thou are not worthy of the name of man.

Alternate:

All Adam's race are members of one frame,
Since all, at first, from the same essence came.
When by hard fortune one limb is oppressed,
The other members lose their wonted rest:
If thou feel'st not for others' misery,
A son of Adam is no name for thee.

Persian original can be seen here. Transliterated (here) as:

Bani aadam aazaye yek digarand
ke dar aafarinesh ze yek gooharand
cho ozvi be dard aavarad roozegaar
degar ozvhaa raa namaanad gharaar
to kaz mehnate digaraan bi ghami
nashaayad ke naamat nahand aadami