Sunan An-Nasai, Chapter: The Book of the Times (of Prayer)

from Anas that a man came to the Prophet (saw) and asked him about the time of the Subh prayer. The following morning he commanded that the Iqamah for prayer be said when dawn broke, and he led us in prayer. The next day when there was light he commanded that the Iqamah for prayer be said and he led us in prayer. Then he said: "Where is the one who was asking about the time for prayer? (It is) between these two times." (Sahih)