Sahih al-Bukhari 4990

Ubayd Allah ibn Musa related to us, from Israil, from Abu Ishaq, from al-Bara, who said: When ‘Those believers who stay behind are not equal to those who strive in the path of Allah’ was revealed, the Prophet (peace and blessings be upon him) said, ‘Call Zayd for me and let him bring the writing tablet and inkpot and shoulder blade—or the shoulder blade and inkpot.’ Then he said, ‘Write: “Those who stay behind are not equal…”’ Amr ibn Umm Maktum, who was blind, was behind the Prophet. He said, ‘Messenger of Allah, what do you command me, for I am blind?’ Then this phrase was revealed in its place: ‘Those believers who stay behind are not equal to those who strive in the path of Allah, except those with a disability.’