When I am not joining for Moulood he asks what is the reason for not joining and where is the proof?

All perfect praise be to Allah, The Lord of the Worlds. I testify that none is worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

Had there been a specific ruling (or distinct status) related to the birthday of the Prophet the knowing of which entails offering a certain prescribed act of worship, then such ruling would have been reported to us in the Sunnah and the practice of the rightly-guided caliphs. Since this was not the case, despite the need for this (distinction to be stated and clarified) and the absence of any obstacle preventing from doing so, then it is deduced that celebrating the birthday of the Prophet falls under the category of blameworthy innovation in religion about which the Prophet said: “Whoever introduces into this matter of ours (Islam) something which does not belong to it in origin, it will be rejected by Allah, The Almighty.” [Al-Bukhaari and Muslim]

The Maaliki scholar Ibn Al-Haajj wrote, “Whoever throws a banquet with the sole intention of celebrating the birthday of the Prophet and invited people over, his act is considered a religious innovation.” [Al-Madkhal]

Knowing that, you come to realize that refraining from partaking in such celebrations is the right thing to do, in order to reject religious innovations and to stay away from introducing into Islam something that does not belong to it. The Prophet warned us against such practices and urged us to adhere to his Sunnah and the practice of the rightly-guided caliphs. He said: “Whosoever among you shall live after me, will witness substantial controversy. So, adhere to my Sunnah and that of the rightly-guided Caliphs after me. Hold fast to it, and cling to it with the molars. Beware of newly-invented matters (in religion), for every newly-invented matter (in religion) is Bid‘ah (religious innovation), and every Bid‘ah is sheer misguidance.” [Abu Daawood, At-Tirmithi, and Ibn Maajah] [At-Tirmithi and Al-Albaani: authentic]‎

Allah knows best.