Fiqh-us-Sunnah Volume 5, Hajj and ‘Umrah, Fiqh 5.028.

Ibn ‘Abbas, Ibn ‘Umar, Jabir and Ash-Shafi’i are of the opinion that donning the garb of ihram for Hajj is not valid outside its months. (Anyone putting on ihram before the months of Hajj may, however, perform ‘Umrah, but this cannot be a substitute for his ihram for Hajj)

Ibn ‘Umar said, “The months of Hajj are Shawwal, Zhul-Qui’dah and ten days of Zhul-Hijjah.” Ibn ‘Abbas said: “The sunnah is to declare one’s intention and to put on ihram with the intention of performing Hajj during the months of Hajj.” (Bukhari)

Ibn Jarir reported from Ibn ‘Abbas that he said: “To put on ihram for Hajj is not valid except in the months of Hajj.”

The Hanafites, Malik, and Ahmad are of the opinion that to put on ihram for Hajj before its months is permissible but disliked. Ash-Shawkani, however, preferred the first opinion. He observes: “Allah, the Exalted One, has named specific months for Hajj rites, of which ihram is one. So whosoever claims that it is permissible to put on ihram even before these months must produce evidence to support his claim.”