It is narrated from Hazrat Aisha that the Prophet (peace be upon him), after performing two rak'ahs before Fajr, would sometimes lie down on his right side.
Hadith Referenceمسند احمد / مسند النساء / 24904
Hadith Gradingحکم دارالسلام:حديث صحيح دون قوله: ربما، وقد انفرد بها أبو المؤمل