Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said that the Mother of the Believers, Sayyidah Hafsah (may Allah be pleased with her), told him that when the mu'adhdhin finished the adhan for the morning prayer and the dawn appeared, he (peace and blessings be upon him) would pray two light rak'ahs before the iqamah for the prayer was called.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4536