What is the ruling on growing nails long, with evidence?

Lengthening nails is contrary to the Sunnah (definition), and the wisdom behind trimming them is cleanliness and avoiding microbes. Furthermore, long nails may prevent water from reaching the skin during purification, causing harm. Al-Bayhaqi and Al-Tabarani narrated from Abu Ayyub Al-Azdi that a man came to the Prophet, peace and blessings be upon him, asking him about news of the heavens. He replied: "One of you asks about news of the heavens while his nails are like those of birds, accumulating impurity and filth."

Lengthening nails is considered reprehensible (makrooh) by the majority of scholars, and the reprehensibility intensifies if it exceeds forty nights. Some scholars even consider it forbidden, citing the Hadith (definition) of Anas, may Allah be pleased with him: "The Messenger of Allah, peace and blessings be upon him, set a time limit for us regarding trimming the mustache, clipping the nails, plucking the armpit hair, and shaving the pubic hair: that we should not leave any of these for more than forty nights." The meaning of "setting a time limit" is not to delay these actions beyond their due time, rather than an absolute permission to delay them for forty days.