Praise be to Allah.
Allaah says (interpretation of the meaning):
The believers are nothing else than brothers (in Islamic religion) [al-Hujuraat 49:10]
And He describes them (interpretation of the meaning):
severe against disbelievers, and merciful among themselves [al-Fath 48:29]
The Prophet (peace and blessings of Allaah be upon him) said: The relationship of the believer towards the people of faith is like that of the head to the body. The believer feels the pain of the people of faith just as the body feels the pain of the head.
(Narrated by Imaam Ahmad).
Ibn al-Qayyim (may Allaah have mercy on him) neatly summed up the ways in which a believer can support his fellow-believer. He said:
There are different ways of supporting a fellow believer: with ones wealth, with ones position of authority or power, by means of physical service, by offering sincere advice and guidance, by making supplication (duaa) and asking for forgiveness for them, and by feeling their pain. The level of ones support will be affected by the level of one's faith. The weaker a person's faith, the weaker his support will be; the stronger the faith, the stronger the support. The Messenger of Allaah (peace and blessings of Allaah be upon him) was the greatest of all people in his support for his companions, so the more closely his followers follow him, the greater will be their support for their fellow-believers. (al-Fawaaid, 1/171)
Al-Khaleel ibn Ahmad was walking with a companion of his when his companions shoe broke, so he carried his shoes and walked barefoot. Al-Khaleel took off his own shoes and carried them and walked barefoot. His companion asked him why he was doing that, and he said, I am showing sympathy for you in your being barefoot.