Malik related to me from Abdullah ibn Dinar that Abdullah ibnUmar said, "When we took an oath of allegiance with him to hear andobey, the Messenger of Allah, may Allah bless him and grant him peace,said to us, 'In what you are able.' "
USC-MSA web (English) reference: Book 55, Hadith 1
Arabic reference: Book 55, Hadith 1811