Imam Tirmidhi, in his book even gives the title of one of this chapters, the name “Milaad-un-Nabee”. (Tirmidhi, shamaa’il)
Imam Waaqdi (A second century Scholar) also wrote a book entitled “Milaad-un-Nabi”. The original copy can be found in Cairo.
Allama Ibn Al-Jawzi, wrote a book called “Milaad-un-Nabi” in which he gives very strong proofs permitting the celebration of Milaad.