In the narration of Muslim, 'Aishah (May Allah be pleased with her) said: When the Messenger of Allah (PBUH) used to hear the Adhan (of Fajr prayer) he would perform two supererogatory Rak'ah prayer and would make them short.
Commentary: "Takhfif" here means that the Prophet (PBUH) used to shorten the standing, recitation, bowing, prostration, etc., in the two Rak'ah of Fajr prayer because soon after performing them he had to lead the Fajr prayer in which he would prolong his recitation. He would also offer these two Sunnah soon after the daybreak or Adhan, which gives an idea of the preparation he made for them.