Musnad Ahmad 291:1

Hushaym narrated to us, from Ibn Abi Khalid and Zakariya, from al-Sha’bi, who said: Urwah ibn Mudarris informed me, saying: I came to the Prophet, peace and blessings of Allah be upon him, while he was at Jam’ (Muzdalifah), and I said: "O Messenger of Allah, I have come to you from the mountains of Tayy; I have exhausted myself and worn out my riding beast. By Allah, I have not left a mountain pass except that I stood upon it, so is there any Hajj for me?" He said: "Whoever witnesses this prayer of ours with us—meaning the Fajr prayer—at Jam’, and stands with us until we depart from it, and he had already departed from Arafat before that, by night or by day, then his Hajj is complete and he has fulfilled his rites."