Shama-il Tirmidhi Chapter 039, Hadith Number 004 (251).

Aswad bin Yazeed Radiyallahu 'Anhu says he enquired from 'Aayeshah Radiyallahu 'Anha regarding the salaah of Rasulullah Sallallahu 'Alayhi Wasallam at night. She replied: "Rasulullah Sallallahu 'Alayhi Wasallam slept (after 'eshaa) for the first half portion of the night. He then awakened (and performed the tahajjud prayers) till the time of suhur (sehri), thereafter he performed the witr salaah. He then went to his bed. If he had a desire, he went to his wife. When he heard the adhaan, he got up. If he was in a state of janaabah (requiring ghusl) he performed ghusl. If not, he performed wudu and went for salaah".

Commentary
The physicians say the best time for intercourse is the last part of the night. At that time the stomach is mild and both parties are fresh with regained strength. In the first part of the night, the stomach is full. In such a state it is detrimental to have intercourse. It is more detrimental when one is hungry. In the last portion of the night the stomach is not too full, nor is it very empty so that one feels hungry. All this is medically speaking. In the shari'ah it is permissible to have intercourse with the wife at other times. This is evident that Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam had relations in the first part of the night and at different times of the day. Some mashaa-ikh have written that if one has intercourse with the wife at the time of salaah, resulting in her falling pregnant, the child from that union will be disobedient to the parents. (Because of the disobedience of the parents in not fulfilling salaah).