Abdullah bin Abbas reported,

"You who believe, fasting has been prescribed for you as it was prescribed for those before you" [Baqarah 2:183]. In the time of the Prophet ﷺ, once people had prayed the night prayer, food, drink, and intimacy with women became forbidden to them, and they fasted through until the following night. Then one man wronged himself and slept with his wife after he had prayed the evening prayer and had not broken his fast. So Allah willed to make the matter easy for those who remained, granting them a concession and a benefit, and He, glorified be He, said, "Allah knows that you were deceiving yourselves" [Baqarah 2:187]. This was among the things by which Allah benefited people, granting them a concession and making matters easy for them.

عن ابن عباس

{يأيها الذين ءامنوا كتب عليكم الصيام كما كتب على الذين من قبلكم} [سورة البقرة: ١٨٣]، فكان الناس على عهد النبي ﷺ إذا صلوا العتمة: حرم عليهم الطعام والشراب والنساء، وصاموا إلى القابلة. فاختان رجل نفسه فجامع امرأته وقد صلى العشاء ولم يفطر. فأراد الله أن يجعل ذلك يسرا لمن بقي ورخصة ومنفعة، فقال سبحانه: {علم الله أنكم كنتم تختانون أنفسكم} [البقرة: ١٨٧] وكان هذا مما نفع الله به الناس ورخص لهم ويسر.