Bismillah was shalatu was salamu 'ala rasulillah, amma ba'du,
when people junub at night ramadan, either because of wet dreams or because of intercourse, or because masturbation, then not madi until entering dawn, whether fasting is legitimate. Cases that often occur, they junub at night and overslept, then wake up already entered the dawn.
Because of ignorance, there are some people who are reluctant to fast because they have not shower junub when entering dawn. What's worse, some do not pray at dawn because they continue to sleep until the morning. Yet all these actions, leaving prayers or not fasting for no reason, is a very big sin. Meanwhile, not yet bathing when entering at dawn, NOT a reason that allows a person to leave the fast. And abandoning fasting without the proper correctness gets very loud, as described in: The Law of Breaking the Fast for No Reason
Not Bathing When Entering Dawn
It is not a valid requirement of fasting, a person must be pure of hadats big or small. This is different from the prayer or thawaf in the Ka'aba. People who want to pray or thawaf, must be pure of hadats big and small. And if there is hadats in the middle of prayer then the prayer is canceled. Another case with the fasting, holy of hadats is not a valid requirement of fasting. Can not we imagine if fasting must be holy hadi hadats, surely all those who fast will be very hassle. Because they should not fart or dispose of water during fasting.
Therefore, people who junub and have not bathed until dawn, no need to worry, because this kind does not affect his fast. The main point of the matter is the hadith of Aisha and Umm Salamah radhiallahu 'anhuma; they tell,
كان يدركه الفجر وهو جنب من أهله, ثم يغتسل ويصوم
"The Prophet sallallaahu 'alaihi wa sallam entered the time of dawn, while he was junub because of his relationship with his wife. Then, he showered and fasted. "(Narrated by Bukhari 1926 and Turmudzi 779).
At-Tumudzi after mentioning this hadith, he said,
والعمل على هذا عند أكثر النبي صلى الله عليه وسلم, وغيرهم, وهو قول سفيان, والشافعي, وأحمد, وإسحاق
This is understood by the majority of scholars among the Companions of the Prophet sallallaahu 'alaihi wa sallam and others. And this is the opinion of Sufyan At-Tsauri, As-Syafi'i, Ahmad, and Ishaq bin Rahuyah. (Sunan At-Turmudzi, 3/140).
Can Sahur in Junub Condition?
When a man junub wakes up at the end of the night, he is in a state to choose between bathing and dying, what should come first?
From the above explanation, we have the conclusion that junub bath should not be done before dawn. One may bathe after the dawn, and the fast will be valid. While dawn, the last border is dawn. One can not be duh after entering at dawn. Considering this, a person may be able to delay bathing and may not delay the meal. Therefore, what he probably did was put the dawn first and put the bath on hold.
Only, before eating meal, it is recommended to perform ablution first. As the information of 'A'isha (may Allaah be pleased with him) said,
كان رسول الله صلى الله عليه و سلم إذا كان جنبا فأراد أن يأكل أو ينام توضأ وضوءه للصلاة
"When the Prophet sallallaahu 'alaihi wa sallam is in junub condition, then he wants to eat or sleep, he performs ablution as ablution when going to pray." (H.r. Muslim, 305).
If You Want to Pray Fajr, Bath First
Note, do not let the condition when fasting junub make you leave the morning prayer, due to lazy bathing. Because leaving prayer is a very big sin. Before praying, take a bath first, because this is a legitimate requirement of prayer.
Allah says,
وإن كنتم جنبا فاطهروا
"If you are in a state of junub, wash .." (Surah Al-Maidah: 6)
So, hopefully useful.