Musnad Ahmad 32:1715

'Affan narrated to us, Hammad narrated to us, he said: Thabit informed us, from Anas ibn Malik, that al-'Adba' [the Prophet's she-camel] would not be outrun. Then a Bedouin came on a young camel of his and raced her, and the Bedouin outran her. This seemed difficult for the companions of the Messenger of Allah, peace and blessings of Allah be upon him. So the Messenger of Allah, peace and blessings of Allah be upon him, said: "It is a right upon Allah that He does not raise anything of this world except that He lowers it."