Sahih Bukhari Volume 4, Book 52, Hadith Number 252.

Narated By Jabir bin ‘Abdullah : When it was the day (of the battle) of Badr, prisoners of war were brought including Al-Abbas who was undressed. The Prophet looked for a shirt for him. It was found that the shirt of ‘Abdullah bin Ubai would do, so the Prophet let him wear it. That was the reason why the Prophet took off and gave his own shirt to ‘Abdullah. (The narrator adds, “He had done the Prophet some favor for which the Prophet liked to reward him.”)