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