Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates: It is the Sunnah that ihram for Hajj be entered only in the months of Hajj.
Al-Tabarani recorded it in al-Mu'jam al-Kabir. Its chain contains al-Hajjaj bin Artah, who has been criticised, though he has been declared trustworthy.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الحج / 5331