It was started by raafidi shia'as
فإنَّ ممَّا أحْدَثه الناسُ في القُرونِ المتأخِّرةِ بعدَ القرونِ الثَّلاثةِ الأُولى المُفضَّلةِ: الاحتفالَ بيومِ وِلادةِ النبيِّ صلَّى الله عليه وسلَّم، وأمَّا قرنُ الصَّحابةِ والتابِعين ومَن جاءَ بعدَهم، فلم يكُن أحدٌ فيه يَحتفِلُ بمولدِ النبيِّ صلَّى الله عليه وسلَّم، لا صحابتُه الأبرار، ولا مَن جاء بَعدَهم مِن العُلماءِ والأئمَّةِ المتبوعِين الأخيار، لا مِن أئمَّة الفِقهِ كأبي حَنيفةَ ومالكٍ والشافعيِّ وأحمدَ، ولا مِن المُحدِّثين كالبُخاريِّ ومُسلِمٍ وغيرِهما، وإنَّما أُحدِثَ هذا الاحتِفالُ البِدعيُّ في أواخِر القرنِ الرابعِ الهِجريِّ، وأوَّلُ مَن أحْدَثه وابتدَعه هم الرافضةُ العُبَيديُّون (الذين يُسمَّوْن زُورًا وتَلبيسًا بالفاطميِّين)
I think this opinion is more sound if you wish to be on the safer side. Celebrating the birthday of the prophet Muhammed SAW seems too close to what the Christians and Jews do for their holy figures.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]