Tirmidhi

Commentary:
Note:
(According to the research of some scholars, the narration about the Prophet sallallahu alayhi wa sallam lying down is authentic, and the hadith with the Prophetic statement is defective. For details, see:
I‘lam Ahl al-‘Asr bi-Ahkam Rak‘atay al-Fajr,
authored by:
Muhaddith Shams al-Haq Azimabadi;
Shaykh al-Islam Ibn Taymiyyah wa Juhuduhu fi al-Hadith wa ‘Ulumihi,
authored by:
Abdul Rahman bin Abdul Jabbar al-Faryawi.)

1261. Commentary:
On this issue, «اعلام اهل العصر بأحكام ركعتي الفجر» the eminent and detailed book by Allamah Shams al-Haqq Dehlawi exists. He states that lying on the right side after the Sunnah prayers of Fajr is Sunnah, whether one has performed Tahajjud or not. And its narrators are Aisha, Abu Hurairah, Abdullah ibn Abbas, and Abdullah ibn Amr (radi Allahu anhum ajma'in).

Takhrij:
«أخرجه أبوداود، الصلاة، باب الاضطجاع بعدها، حديث:1261، والترمذي، الصلاة، حديث:420، وأحمد:2 /415.*سليمان بن مهران الأعمش مدلس وقد عنعن، وعنعنة المدلس ضعيفة علي الراجح إلا إذا صرح بالسماع في طريق آخر أو توبع من مقبول الحديث.»©Explanation:
➊ The aforementioned narration has been declared weak in its chain of transmission by our esteemed researcher, whereas other scholars, based on supporting evidences, have considered the issue mentioned therein to be authentic. As the previous hadith from Bukhari also supports and confirms this issue. For details, see: (Al-Mawsu‘ah al-Hadithiyyah, Musnad al-Imam Ahmad: 15/218, Hadith: 9368). Therefore, despite the aforementioned narration being weak in its chain, it is actionable due to other supporting evidences.

➋ From two hadiths, it is established as Sunnah to recline briefly on the right side for rest after performing the Sunnah prayers of Fajr. One hadith establishes the practice of the Prophet (sallallahu alayhi wa sallam), and another establishes his command. On this basis, according to the literalists (Ahl al-Zawahir), this reclining is obligatory, and whoever deliberately does not act upon it, his Fajr prayer is not valid. However, Imam Nawawi rahimahullah states that the correct opinion is that it is Sunnah. Some have considered it disliked (makruh), but this view is absolutely incorrect in the face of an authentic hadith.

➌ The commentator of Sunan Abu Dawud, Mawlana Shams al-Haq Dehlawi rahimahullah, has discussed this issue in great detail in I‘lam Ahl al-‘Asr bi-Ahkam Rak‘atay al-Fajr. He states that reclining on the right side after the Sunnah prayers of Fajr is Sunnah, whether one has performed Tahajjud or not.