When anyone of you prays two Rak‘ahs before the dawn prayer, he should lie on his right side
Authentic hadithعن أبي هريرة رضي الله عنه ، قال: قال رسول الله صلى الله عليه وسلم : «إذا صلى أحدكم الركعتين قبل الصبح، فليضطجع على يمينه»، فقال له مروان بن الحكم: أما يجزئ أحدنا ممشاه إلى المسجد حتى يضطجع على يمينه، قال عبيد الله في حديثه: قال: لا، قال: فبلغ ذلك ابن عمر، فقال: أكثر أبو هريرة على نفسه، قال: فقيل لابن عمر: هل تنكر شيئا مما يقول؟ قال: «لا، ولكنه اجترأ وجبنا»، قال: فبلغ ذلك أبا هريرة، قال: «فما ذنبي إن كنت حفظت ونسوا».
Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "When anyone of you prays two Rak‘ahs before the dawn prayer, he should lie on his right side." Marwān ibn al-Hakam said to him: "Is it not enough that one of us walks to the mosque until he lies on his right side?" According to the version of ‘Ubaydullāh, he (Abu Hurayrah) replied: 'No.' This statement (of Abu Hurayrah) reached Ibn ‘Umar. He said: "Abu Hurayrah exceeded the limits on himself." He was asked: "Do you approve of what he says?" He replied: "No, but he dared and we showed cowardice." This (criticism of Ibn ‘Umar) reached Abu Hurayrah. He said: "What is my sin if I committed it to memory but they forgot?"
Explanation
This Hadīth states that it is from the Sunnah of the Messenger (may Allah's peace and blessings be upon him) to lie down on one’s right side after praying the two Rak‘ahs before Fajr. He is to remain in this position until the Iqāmah for the prayer is called. The wisdom behind lying on one’s right is so that the person does not fall asleep, since the heart is located on the left side of the chest. When a person lies down on his left side, he is more prone to falling asleep, because he will feel more relaxed.
Source · Narrated by At-Termedhy - Narrated by Abu Daoud - Narrated by Ahmad
From the Hadith Encyclopedia (hadeethenc.com).