It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) used to offer the Sunnah of Fajr while the call to prayer was still being heard in his ears.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 6090
Hadith Gradingحکم دارالسلام:إسناده صحيح ، خ: 995، م: 749.