Sunan An-Nasai, Chapter: The Book of Salah

"We were with the Messenger of Allah (saw) and he said: 'Will yu not pledge to the Messenger of Allah (saw)?' And he repeated it three times. So we stretched forth our hands to give our pledge. We said: 'O Messenger of Allah, we are willing to give you our pledge, but on what?' He said: 'That you will worship Allah and not associate anything with him, and (offer) the five daily prayers.' And he said, very quietly: 'And you will not ask the people for anything.'" (Sahih)