Musannaf Ibn Abi Shayba 28:1031

Sahl ibn Yusuf narrated to us, from Humayd, from Bakr, who said: Two men saw a gazelle while they were in the state of ihram. They made a pact regarding it and made a wager. One of them threw a stick at it and broke it. They came to Umar, and Abd al-Rahman ibn Awf was at his side. He said to Abd al-Rahman, "What do you say?" He replied, "This is gambling, even if it were a race."