Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says that when the verse "O you who believe, fasting has been prescribed for you as it was prescribed for those before you" [Surah al-Baqarah: 183] was revealed, the people in the time of the Prophet (peace and blessings be upon him) would fast; when they had prayed Isha, food, drink and women (marital relations) became forbidden for them until the next fast, and the next fast would begin. Then a man betrayed himself and had intercourse with his wife after he had prayed Isha and had not broken his fast. So Allah Almighty willed to make it easy for the rest and to make it a concession and a benefit for them, and He said: "Allah knows that you used to betray yourselves, so He turned to you in mercy and pardoned you" [Surah al-Baqarah: 187]. So Allah Almighty made this a benefit for the people, and made it easy for them by granting them a concession.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7901