Sahih Bukhari Volume 6, Book 60, Hadith Number 116.

Narrated By Zaid bin Thabit : That the Prophet dictated to him: “Not equal are those of the believers who sit (at home) and those who strive and fight in the Cause of Allah.”

Zaid added: Ibn Um Maktum came while the Prophet was dictating to me and said, “O Allah’s Apostle! By Allah, if I had the power to fight (in Allah’s Cause), I would,” and he was a blind man. So Allah revealed to his Apostle while his thigh was on my thigh, and his thigh became so heavy that I was afraid it might fracture my thigh. Then that state of the Prophet passed and Allah revealed: “Except those who are disabled (by injury or are blind or lame etc).”