Musnad Ahmad 409:0

Narrated by Abd al-Rahman ibn Khabbab al-Sulami: The Prophet (peace be upon him) addressed the people and encouraged them to equip the Army of Hardship (Jaysh al-Usra). Uthman ibn Affan stood up and said, "O Messenger of Allah, I pledge one hundred camels with their saddles and equipment for the sake of Allah." Then the Prophet (peace be upon him) encouraged the people again. Uthman stood up and said, "O Messenger of Allah, I pledge two hundred camels with their saddles and equipment for the sake of Allah." Then the Prophet (peace be upon him) encouraged the people again. Uthman stood up and said, "O Messenger of Allah, I pledge three hundred camels with their saddles and equipment for the sake of Allah." I saw the Messenger of Allah (peace be upon him) descending from the minbar, saying, "It will not harm Uthman what he does after this. It will not harm Uthman what he does after this."