Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.124.

Abu Hurairah reported that the Messenger of Allah, upon whom be peace, said: “Do not perform al-wisal.” He said that three times and the people said to him: “But you perform al-wisal, O Messenger of Allah!” He said: “You are not like me in that matter. I spend the night in such a state that Allah feeds me and gives me to drink.. Devote yourselves to the deeds which you can perform.” This is related by al-Bukhari and Muslim.

The scholars say this prohibition implies that the act is disliked. Ahmad and Ishaq say that it is allowed to fast until the time of the pre-dawn meal as long as it is not a hardship on the one fasting. This opinion is based on what al-Bukhari recorded on the authority of Abu Sa’id al-Khudri: “The Messenger of Allah said: ‘Do not make al-wisal. If one of you insists on making al-wisal, he may continue his fast [after sunset] until the time of the pre-dawn.