Musannaf Ibn Abi Shaybah

Hasan says regarding the person who assumes Ihram for Umrah and when he arrives, the people are staying at Arafat: if he is certain that he can reach Makkah (i.e., going will not cause him to miss Hajj), then he should become halal from his Umrah (i.e., complete the Umrah in Makkah and become halal). Otherwise, he should recite the Talbiyah for Hajj and perform two Tawafs.