He did ghusl then he realized that the water had not reached his foot

Firstly: 

One of the conditions of wudoo’ and ghusl, that is essential in order for them to be  correct and acceptable, is removing everything that may prevent the water from reaching the skin, such as creams, dyes and colours that have any substance that could prevent the water from reaching the skin. 

Imam al-Shaafa’i (may Allaah have mercy on him) said in his book al-Umm (1/44): 

If there is on him any mastic or anything thick that will prevent the water from reaching the skin, his washing of that part for wudoo’ is not valid, unless he removes it or removes enough so that he knows that there is no barrier to the water touching the skin. End quote. 

This has been explained in the answer to question no. 69817.  

Secondly: 

If the person who is doing ghusl leaves out a part of his body and the water does not reach it, or there is something preventing the water from reaching part of his body, then he realizes that after some time has passed, should he repeat the entire ghusl or is it sufficient to wash the part that the water did not reach? 

This question is based on the ruling on continuity in washing the parts of the body in wudoo’. Continuity means that the parts are washed one after the other with no lengthy interruption in between. 

The majority of scholars are of the view that continuity is not essential in ghusl. 

It says in al-Mawsoo’ah al-Fiqhiyyah (11/100-101): 

It is not essential to wash the parts in order and one after the other according to the majority of fuqaha’. 

Al-Layth said: Continuity is essential. 

Different opinions were narrated from Imam Ahmad, but the view that takes precedence according to his companions is that continuity is essential. This view was also favoured by some of the companions of Imam al-Shaafa’i. 

According to the majority view: If a person did not wash a part of the body, or a spot on that part, he may wash it on its own later on, whether it is a long time or a short time afterwards. End quote. 

See also al-Mughni (1/220) and Minah al-Jaleel (1/125). 

Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) favoured the view of the majority in Sharh al-‘Umdah (1/208-209). And quoted a great deal of evidence to support it, including the following: 

1 – The hadeeth narrated by Ibn ‘Abbaas from the Prophet (peace and blessings of Allaah be upon him): that he noticed a spot that the water did not reach after doing ghusl and he wrung out his (wet) hair onto it.

Narrated by Ibn Majaah (663); classed as da’eef by Ibn Hajar in al-Diraayah (1/55), Ibn al-Jawzi in al-‘Ilal (1/347), al-Busayri in Misbaah al-Zujaajah and al-Albaani in Da’eef Ibn Majaah. 

2 – It was narrated that ‘Ali (may Allaah be pleased with him) said: 

A man came to the Prophet (peace and blessings of Allaah be upon him) and said, “I did ghusl to cleanse myself from janaabah, and I prayed Fajr, then I noticed a spot the size of a fingernail that the water did not reach.” The Messenger of Allaah (S) said: “If you had wiped it that would have been sufficient for you.” 

Narrated by Ibn Majaah (664); classed as da’eef by al-Busayri in Misbaah al-Zujaajah and al-Albaani in Da’eef Ibn Majaah 

3 – The Prophet (peace and blessings of Allaah be upon him) commanded the one who was junub to do wudoo’ as for prayer if he wanted to sleep, eat or have intercourse again. The companions of the Messenger of Allaah (peace and blessings of Allaah be upon him) would converse in the mosque after doing wudoo’ even if they were junub. Were it not for the fact that wudoo’ reduces the level of janaabah, there would have been no benefit in doing wudoo’ after becoming junub. If the level of janaabah may be reduced, then janaabah is divisible, and if it is divisible them then it may be separated, unlike wudoo’ which is not divisible, rather impurity is not removed from one part (in wudoo’) unless it is removed from all parts. 

4 – Ibn ‘Abbaas said concerning one who forgets to rinse out his mouth and nose when doing ghusl from janaabah and prays: He should go and rinse out his mouth  and nose and repeat the prayer. Narrated by Sa’eed in his Sunan. 

Also narrated by al-Daaraqutni in his Sunan (1/16). A similar report was narrated from a number of the Salaf, as stated in Musannaf Ibn Abi Shaybah (1/224-225). 

5 – Continuity is connected to sequence, and sequence has to do with the order in which two or more parts are to be washed, and the body of the person who is junub is like one part.  

6 – Allowing for ghusl to be divided is needed a great deal and may be required for one who is overweight, as he may forget a spot or two or the roots of his hair, and it may be too difficult to repeat it, but that rarely happens with wudoo’, which it is easy to repeat, so there is a difference between the two.  

End quote. 

Based on this, if a person misses a part of his body and the water did not reach it when he did ghusl, or there was some barrier on it that prevented the water from reaching the body, it is sufficient for him to wash only the part that was missed, and he does not have to repeat ghusl. 

Imam al-Shaafa’i said in al-Umm (2/88): 

If he misses a spot on his body – large or small – and prays, he should wash the part that he missed on his body and repeat the prayer after washing it. End quote. 

See also question no. 9066 and 50320.