Lying on One's Right Side before Fajr Prayer
»'Aisha (may Allah be pleased with her) narrated that:
"Allah's Apostle (Peace and blessings be upon Him) used to pray two light rak'at (one unit of prayer) before the morning obligatory prayer (al-Fajr), when the day dawned and the Mu'adh-dhin had finished the Adhan. He then would lie on his right side until the Mu'adh-dhin came to pronounce the Iqama. «([1])
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([1]) Sahih al-Bukhari – Kitab al-Adhan (626), Sahih Muslim – Kitab Salatu al-Musafereen