Praise be to Allah.
Al-Tabari (may Allaah have mercy on him) said in his Tafseer: Fitrah: the deen (way or religion) of Allaah. Al-Tabaris tafseer (commentary) of the aayah (interpretation of the meaning): [Iblees said] and indeed I will order them to change the nature created by Allaah. [al-Nisa 4:119]
Concerning the aayah (interpretation of the meaning), So set your face steadily and truly to the Faith: (establish) Allaahs handiwork according to the pattern (fitrah) on which He has made mankind: no change (let there be) in the work (wrought) by Allaah: that is the standard Religion: but most among mankind understand not. [al-Room 30:30 Yusuf Alis translation], it was reported from some of the scholars of tafseer that the phrase the pattern (fitrah) on which He has made mankind means the design of Allaah according to which He has created mankind.
This word (fitrah) was also mentioned in the hadeeth narrated by Abu Hurayrah who said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: Five things are part of the fitrah: removing the pubic hair, circumcision, trimming the moustache, plucking the armpit hairs, and trimming the nails. (Reported by al-jamaaah )
What is meant by these five things being part of the fitrah is that when they are done, this is in accordance with the natural pattern on which Allaah made mankind and urged them to follow, so that they will be better and more perfect This is the ancient sunnah (way) which was followed by all the Prophets and which was enjoined by all the laws they brought. It is a natural and innate way.(Al-Shawkaani, Nayl al-Awtaar, Baab Sunan al-Fitrah).