Imam Syihabuddin Ahmad Ibnu Hajar Al-Haitami Asy-Syafi'i said in his book Ni’matul Kubra 'Alal 'Alam Fi Maulidi Saidi Waladi Adam that Saiyidina 'Umar said "Whoever honors the occasion of the Prophet's maulid has indeed revived Islam." The source of this is from an unnamed book.