Al-Matalib al-'Aliyah 1160 — The time-based miqat

It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that he said: "It is part of the pure Sunnah that the ihram for Hajj be entered only during the specified months of Hajj."
According to the Imams and commentators of hadith, a Companion's statement that "this practice is Sunnah" in fact points to the blessed way of the Messenger of Allah (peace and blessings be upon him), and it is in the rank of a marfu' hadith in ruling; from it comes the juristic guidance that beginning the intention and ihram of Hajj before Shawwal, Dhu al-Qa'dah and the first days of Dhu al-Hijjah is not in accordance with the Sunnah.