Musnad Ahmad 937:220

Abu al-Mughirah narrated to us, Safwan narrated to us, from Abu al-Yaman and Abu al-Muthanna, that Abu Dharr said: The Messenger of Allah (may Allah bless him and grant him peace) took my pledge of allegiance five times, and he took a solemn covenant from me seven times, and he called witnesses over me nine times, that I would not fear the blame of any blamer for the sake of Allah. Abu al-Muthanna said: Abu Dharr said: The Messenger of Allah (may Allah bless him and grant him peace) called me and said: "Will you give a pledge of allegiance, and for you is Paradise?" I said: "Yes." And I stretched out my hand. The Messenger of Allah (may Allah bless him and grant him peace) said, while stipulating a condition upon me: "That you do not ask people for anything." I said: "Yes." He said: "Not even your whip if it falls from you, until you dismount for it and take it yourself."