Can We Delay Ghusl after Intercourse?

It is permissible to delay ghusl until the time of Fajr but it is mustahabb (recommended) for the husband and wife to do wudu before they sleep. This was the view of the majority of scholars, and al-Nawawi reported that there was scholarly consensus on this point. (Al-Kafi, 1/173; al-Muhadhdhab, 1/33; al-Mughni, 1/229)

Their evidence is the hadith of `Aishah (may Allah be pleased with her) whose authenticity is agreed upon, in which she narrated that when the Messenger of Allah (peace and blessings of Allah be upon him) wanted to sleep whilst he was in a state of impurity , he would wash his private parts and do wudu as for prayer. (Narrated by al-Jama`ah)

They also use as evidence the report narrated by Ibn ‘Umar (may Allah be pleased with him) who said that ‘Umar said, “O Messenger of Allah, may any of us sleep when he is in a state of impurity?” He said, “yes, if he does wudu.” (Agreed upon).

The wisdom behind doing wudu is that it reduces the impurity and is better for the body.

The Zahiriyyah and Ibn Habib among the Malikis thought that it was obligatory for the person who is in a state of impurity to do wudu, on the basis of the hadith of ‘Umar referred to above. 

For more, please see these answers: 82344 ,   46562 , 415 , and 43307 .

And Allah knows best.