Abu Nadrah (may Allah have mercy on him) said: I asked Sayyiduna Ibn Abbas (may Allah be pleased with them both) about the saying of Allah Almighty: "Not equal are those of the believers who sit (at home), other than those with a disability" [Surah al-Nisa: 95]. He said: What is meant by it are the people of disability, that is, people who in the time of the Messenger of Allah (peace and blessings be upon him) could not take part in jihad with him (peace and blessings be upon him); pains and illnesses had kept them from going, while others did not go despite being healthy. So Allah Almighty declared the sick to be excused in comparison with them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 17818