The fundamental argument of those who support the celebration of Mawlid an-Nabawi is that they refer to the celebration of Mawlid an-Nabawi as a 'Bidah Husna' for which they present arguments such as the saying of Umar t, 'What a good Bidah is this?', the gathering of the Qur'aan in one book and the collection of Hadeeth - which are explicitly refuted by Shaikh Salih al-Fawzan in the article. However, one of their main argument still remains unanswered that Allah's Messenger (sallalalhu alaihi wa-sallam) said, 'He, who establishes/initiates a Sunnah Husna (good Sunnah) in Islam, he will have the reward of those who perform it after him until the Day of Judgment ' [Saheeh Muslim]
In a replying to this argument, Shaikh Fawzan says in a fatawa,
"... Concerning the saying of Allah's Messenger (sallalalhu alaihi wa-sallam), '(the afore-mentioned Hadeeth)' This does not prove their stance because Allah's Messenger (sallalalhu alaihi wa-sallam) did not say, 'Whosoever innovates a Bidah, rather he said, 'Whosoever initiates a good Sunnah' and Sunnah is the opposite of Bidah. Sunnah is something that is in agreement with the Qur'aan and the Sunnah and in agreement with the proof.
So (the meaning of the Hadeeth is) he, who performs a Sunnah which is proved by the Qur'aan and the Sunnah; he will have the reward of it (his action) and the reward of those who perform it until the Day of Judgment. (Thus) Whoever revives a Sunnah and performs it for the people, and enlightens it before them, and they follow him then he will have the reward equal to theirs.
[Majmoo Fatawa wa-Rasaa'el Fadhilatus-Shaikh Muhammad Salih al-Uthaimeen (291/2) fatawa no. 346]