Musnad Abu Ya'la al-Mawsili 129:59

On the authority of Anas, may Allah be pleased with him, that a man came to the Prophet, may Allah bless him and grant him peace, and said, "Carry me [provide me with a mount]." He said, "We will carry you on the offspring of a she-camel." He said, "And what would I do with the offspring of a she-camel?" The Prophet, may Allah bless him and grant him peace, said, "And do camels give birth to anything other than she-camels?"