Call to Prayers (Adhaan)

Bukhari :: Book 1 :: Volume 11 :: Hadith 592

Narrated Hafsa:

When the Muadh-dhin pronounced the Adhan for Fajr prayer and the dawn became evident the Prophet ordered a two Rakat light prayer (Sunna) before the Iqama of the compulsory (congregational) prayer.

Source materials are from the University of Southern California MSA site
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