Musnad Ahmad 32:1873

Khalaf ibn al-Walid narrated to us, Khalid ibn Abdullah narrated to us, from Humayd al-Tawil, from Anas ibn Malik, that a man came to the Prophet, peace and blessings of Allah be upon him, and asked him for a mount. The Messenger of Allah, peace and blessings of Allah be upon him, said: "We will carry you on the offspring of a she-camel." The man said: "O Messenger of Allah, what shall I do with the offspring of a she-camel?" The Messenger of Allah, peace and blessings of Allah be upon him, said: "And do camels give birth to anything other than she-camels?"