Muhammad says that a person asked Hazrat Aisha (may Allah be pleased with her) about the Sunnah prayers of Fajr, so she replied that the Prophet (peace and blessings be upon him) used to perform these two rak'ahs briefly, and I think that in them he used to recite Surah Al-Kafiroon and Surah Al-Ikhlas or similar surahs.
Hadith Referenceمسند احمد / مسند النساء / 25499
Hadith Gradingحکم دارالسلام:حديث صحيح دون قولها: فأظنه كان ... وهذا إسناد ضعيف لانقطاعه، محمد بن سيرين لم يسمع من عائشة