How valid is the ruling that celebrating the Prophet's birthday (Mawlid) is impermissible, based on the principle that drawing near to Allah is only through what He has legislated, and that no verse or hadith has been reported regarding this celebration, citing the hadith "Hold firmly to my Sunnah and the Sunnah of the rightly-guided Caliphs," and Imam Ahmad's prohibition of performing prayers outside their prescribed times due to their contravention of the Sunnah, and warning against the actions of the Sufis as they are one of the sects that deviated from the methodology of the Prophet and his Companions?

Issuing a fatwa without knowledge is forbidden, and the one who does so is sinful. The statement mentioned is generally acceptable, but it has some observations. Your saying: "that Sufis are among the misguided sects threatened with Hellfire" is not correct in its absolute sense, because among those affiliated with Sufism are people of goodness and virtue, and among them are people of innovations, superstitions, and polytheism. Branding them as a misguided sect in general is not sound; rather, among them are those who wrong themselves, those who are moderate, and those who are foremost in good deeds. The term "Sufism" itself carries neither praise nor blame; rather, each individual or group is judged according to their state and their conformity to the truth or their deviation from it. However, the prevalent characteristic of contemporary Sufi orders is deviation. What you mentioned about Imam Ahmad is similar to what Abd al-Razzaq narrated from Ibn al-Musayyab concerning the story of the man who kept repeating prostrations after dawn.