His Ihram is valid if he enters into Ihram with what another person has entered into Ihram with, provided he knows what that person has entered into Ihram with; as narrated by Anas (may Allah be pleased with him) who said: "Ali (may Allah be pleased with him) came to the Prophet (peace be upon him) from Yemen, and he said: 'With what have you entered into Ihram?' He said: 'With what the Prophet (peace be upon him) has entered into Ihram.' He said: 'If it were not that I have with me the sacrificial animals, I would have exited Ihram.'" [In Sahih Bukhari 2: 564, and Sahih Muslim 2: 914]. However, if he enters into Ihram with what another person has entered into Ihram with and does not know what that person has entered into Ihram with, then his Ihram is treated as ambiguous, and he must proceed with one of the two rites, and he may choose which one he wishes before he begins the actions of either one. If he does not specify, the following details apply: If he performs Tawaf, even if it is just one circuit, his Ihram is for Umrah, even if he did not intend the Tawaf for Umrah. If he stands at Arafat before the Tawaf, his Ihram becomes for Hajj, even if he did not intend the standing for Hajj. If he is prevented before the actions or misses the standing or has sexual relations, it becomes specified for Umrah. Refer to: Lubab al-Manasik, p. 119.