مِشْكَاةُ الْمَصَابِيحِ
Mishkat al-Masabih 1190
Prayer
Prayer at Night - Section 1
وَعَنْهَا قَالَتْ: كَانَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِذَا صَلَّى رَكْعَتَيِ الْفَجْرِ اضْطَجَعَ عَلَى شقَّه الْأَيْمن "
She said that when the Prophet (ﷺ) prayed the two rak'as of the dawn prayer he lay down on his right side. (Bukhari and Muslim.)
Mishkat al-Masabih, 1190مُتَّفَقٌ عَلَيْهِ (الألباني)