Mawlid is Bidcah and was never celebrated by the Prophet nor by the sahabah let alone the salaf. It was started by the Shica Raafidah of the Fatimid Dynasty. Before anyone says its still allowed I recommend them to follow the Sunnah of the Prophet which is the following;
Jabir ibn Abdullah reported: The Messenger of Allah, peace and blessings be upon him, would praise Allah in his sermon, as He deserves to be praised, and then he would say, “Whomever Allah guides, no one can lead him astray. Whomever Allah sends astray, no one can guide him. The truest word is the Book of Allah and the best guidance is the guidance of Muhammad. The most evil matters in religion are those that are newly invented, for every newly invented matter is an innovation, every innovation is misguidance, and every misguidance is in the Hellfire.”
Source: Sunan al-Nasā’ī 1578
And if this doesn't convince those who still celebrate this newly invented matter then I recommend to read the following:
It was narrated that Ibn ‘Umar said: “The Messenger of Allaah (peace and blessings of Allaah be upon him) said: ‘Whoever imitates a people is one of them.’”
(Narrated by Abu Dawood, al-Libaas, 3512. Al-Albaani said in Saheeh Abi Dawood, (it is) hasan saheeh. No. 3401).
So it was the Christians who began to celebrate the birthday of Isa the prophet of Allah and now the Muslims celebrate the birthday of their Prophet.
Rasulullah ﷺ said: “You will certainly follow the ways of those who came before you hand span by hand span, cubit by cubit, to the extent that if they enter the hole of a lizard, you will enter it too.’ We said, ‘O Messenger of Allah, (do you mean) the Jews and the Christians?’ He said: ‘Who else?’” [Bukhari & Muslim]
So fear Allah and follow the Sunnah of Muhammad ﷺ.
Allah تعالى said:
This is My path, which is straight, so follow it; and do not follow [other] ways, for you will be separated from His way. This He has instructed you so that you may become righteous. [6:153]