Hadith 1093:2
'Abd al-Razzaq narrated to us, Ibn Jurayj informed us, he said: And I heard 'Abd Rabbih ibn Sa'id, the brother of Yahya ibn Sa'id, narrating from his grandfather, who said: He went out for the morning [prayer] and found the Prophet, peace and blessings of Allah be upon him, in the morning [prayer], and he had not performed the two rak'ahs of the dawn [prayer]. So he prayed with the Prophet, peace and blessings of Allah be upon him. Then, when he finished the morning [prayer], he stood up and performed the two rak'ahs of the dawn [prayer]. The Prophet, peace and blessings of Allah be upon him, passed by him and said: "What is this prayer?" He informed him, and the Prophet, peace and blessings of Allah be upon him, remained silent and went on, and he did not say anything.
حدثنا عبد الرزاق، أخبرنا ابن جريج، قال: وسمعت عبد ربه بن سعيد، أخا يحيى بن سعيد، يحدث عن جده، قال: خرج إلى الصبح فوجد النبي صلى الله عليه وسلم في الصبح، ولم يكن ركع ركعتي الفجر، فصلى مع النبي صلى الله عليه وسلم، ثم قام حين فرغ من الصبح فركع ركعتي الفجر، فمر به النبي صلى الله عليه وسلم فقال: " ما هذه الصلاة؟ " فأخبره، فسكت النبي صلى الله عليه وسلم ومضى ولم يقل شيئا