Male circumcision is among the rites of Islam and is part of the (in Arabic): fitrah, or the innate disposition and natural character and instinct of the human creation.
As-Shawkani said in his book Nayl al-Awtar (1/184):
"What the Prophet (s.A.w.) means by Fitrah is that if these characteristics are followed by a man, he would be described as a man of Fitrah, which Allah (s.w.t.) has gifted his servants with, and encouraged them to follow, so that they attain a high degree of respectability and dignity."
Prophet Muhammad (peace be upon him) said, "Five are the acts quite akin to fitrah:Circumcision, clipping or shaving the pubes, cutting the nails, plucking or shaving the hair under the armpits and clipping (or shaving) the moustache." (Reported in Bukhari & Muslim)
Allah ordered Prophet Muhammad (peace be upon him) to follow the religion of Ibrahim (peace be upon him). When Allah says:
"Then We inspired you: 'Follow the religion of Ibrahim, the upright in Faith'."
And part of the religion of Ibrahim is circumcision.
The Prophet Muhammad (peace be upon him) said: "The Prophet Ibrahim circumcised himself when he was eighty years old and he circumcised himself with an axe." (Related by Bukhari, Muslim & Ahmad.)
Ibn Abbas (r.a.) was asked "How old were you when the Prophet Muhammad (peace be upon him) died?" He replied, "At that time I had been circumcised. At that time people did not circumcise the boys till they attained the age of puberty (Baligh)." (Bukhari)
Most Fuqaha' (Islamic Jurists) say that circumcision is obligatory upon the men and this is the opinion of Jumhur (the majority of the scholars). If it were not obligatory, then Prophet Ibrahim (peace be upon him) would not have troubled himself at such a later stage of his life.
The Time for Circumcision:
During the time of the Prophet Muhammad (peace be upon him) circumcision was done for boys at the time of their Aqiqah (It's a traditional celebration for the birth of a child which involves the sacrifice of an animal in thanks to Allah. That's the short answer) as reported in al-Bayhaq.
Other Ahadith mention it being done later. The details here are not important but it goes without saying that this minor operation is easier on a baby than it is on an older boy. If it is essential, circumcision can be delayed for practical reasons, but it would be sensible to perform circumcision before the boy starts praying regularly due to practical purposes of simplifying Taharah, or being clean.
Abdullah Ibn Jabir (r.a.) and Aisha (r.a.) said:
"The Prophet (peace be upon him) performed the Aqiqah of al-Hasan and al-Hussein (the prophets grandsons) and circumcised them on the 7th. Day." (Related in al-Bayhaq & Tabarani)
Imam Nawawi says:
"circumcision is recommended to be performed on the seventh day of infancy-the day of Aqiqah (Al-Majmu 1/303)
It is not essential for the child to remain as he is when he comes forth from his mother's womb, if there is something that may be done for him that serves a purpose and is enjoined by the pure religion. Such things include shaving his head after he is born, because that is in his best interests. The Prophet of Islam (peace and blessings of Allah be upon him) said: "Remove the harm from him."
The same applies to washing the blood from him and cutting the cord by which he was attached to his mother, and other things which are done to benefit him.
2 – The health benefits:
Dr. Muhammad 'Ali al-Baar (a member of the Royal College of Surgeons in the UK and a consultant to the Islamic Medicine department of the King Fahd Centre for Medical Research in the King Abdul Aziz University in Jeddah) says in his book al-Khitaan (Circumcision):
"Circumcision of newborn boys (I.e., within the first month of life) brings numerous health benefits, including:
1 – Protection against local infection in the penis, which may result from the presence of the foreskin, causing tightening of the foreskin, which may lead to retention of urine or infections of the glans (tip) of the penis – which require circumcision in order to treat these problems. In chronic cases, the child may be exposed to numerous diseases in the future, the most serious of which is cancer of the penis.
2 – Infections of the urethra. Many studies have proven that uncircumcised boys are more exposed to infection of the urethra. In some studies the rate was 39 times more among uncircumcised boys. In other studies the rate was ten times more. Other studies showed that 95% of children who suffered from infections of the urethra were uncircumcised, whereas the rate among circumcised children did not exceed 5%.
In children, infection of the urethra is serious in some cases. In the study by Wisewell on 88 children who suffered infections of the urethra, in 36 % of them, the same bacteria was found in the blood also. Three of them contracted meningitis, and two suffered renal failure. Two others died as a result of the spread of the micro-organisms throughout the body.
3 – Protection against cancer of the penis: the studies agree that cancer of the penis is almost non-existent among circumcised men, whereas the rate among uncircumcised men is not insignificant. In the US the rate of penile cancer among circumcised men is zero, whilst among uncircumcised men it is 2.2 in every 100,000 of the uncircumcised population. As most of the inhabitants of the US are circumcised, the cases of this cancer there are between 750 and 1000 per year. If the population were not circumcised, the number of cases would reach 3000. In countries where boys are not circumcised, such as China, Uganda and Puerto Rico, penile cancer represents between 12-22 % of all cancers found in men; this is a very high percentage.
4 – Sexually transmitted diseases (STDs). Researchers found that the STDs which are transmitted via sexual contact (usually because of fornication/adultery and homosexuality) spread more among those who are not circumcised, especially herpes, soft chancres, syphilis, candida, gonorrhea and genital warts.
There are numerous modern studies which confirm that circumcision reduces the possibility of contracting AIDS when compared to their uncircumcised counterparts. But that does not rule out the possibility of a circumcised man contracting AIDS as the result of sexual contact with a person who has AIDS. Circumcision is not a protection against it, and there is no real way of protecting oneself against the many sexually transmitted diseases apart from avoiding fornication/adultery, promiscuity, homosexuality and other repugnant practices. (From this we can see the wisdom of Islamic sharee'ah in forbidding fornication/adultery and homosexuality).
5 – Protection of wives against cervical cancer. Researchers have noted that the wives of circumcised men have less risk of getting cervical cancer than the wives of uncircumcised men.
Health Benefits taken from: al-Khitaan, p. 76, by Dr. Muhammad al-Baar.
And Allah knows best.