Fatwa by Imaam Bin Baaz (rahimahullaah): Going to bed without ablution or Ghusl after sexual relations
What is the ruling on the one who sleeps, whilst he is in a state of Janaabah and he neither performed ablution nor Ghusl?
It is Makrooh; the Sunnah (is that) he performs ablution. The least is that he performs ablution. The Sunnah is that he performs ablution and then sleep. When the Prophet (sallal-laahu-alayhi-wasallam) used approached his wives (i.e. after sexual relations), he used to wash his private parts and then go to sleep. Perhaps take a bath before that; for if (you) take a bath, then it is more perfect. And if you sleep and take a bath late at night, then there is no harm; just as the Prophet (sallal laahu alayhi wasallam) did.
As for going to sleep without ablution or Ghusl, then this is Makrooh. Indeed Umar (radiyallaahu anhu) asked the Prophet (sallal laahu alayhi wasallam) about that; so he said: “Perform ablution and then sleep.” He (Umar) asked: “Can one of us sleep whilst in a state of Janaabah? He (Prophet) said: “If you perform ablution, then sleep.” Therefore, the initial affair is that you perform ablution and then go to sleep.’’
So what is intended is that it has been legislated for the believer to perform ablution after fulfilling his need (i.e. after sexual relations with his wife). He washes his private parts and performs ablution (similar to the) ablution of prayer, then he goes to sleep. And if he makes Ghusl, then that is more perfect. The Prophet (sallal’laahu alayhi wasallam) did this (i.e. ablution) and that (i.e. Ghusl). Perhaps he made Ghusl before sleeping or performed ablution and slept; then he (alayhis salaatu was-salaam) made Ghusl late at night.
[Source: www.binbaz.org.sa/mat/9346 ]