´Ja'far bin Muhammad bin 'Ali bin Al-Husain narrated from his father, that Jabir bin 'Abdullah said:` "The Messenger of Allah (ﷺ) prayed Subh as soon as he was certain the dawn had appeared."
Explanation & Benefits
Hafiz Muhammad Ameen
544. Commentary: The earliest time for the morning prayer (Salat al-Fajr) without any difference of opinion is true dawn (subh sadiq). By subh sadiq is meant that white strip of light which spreads horizontally along the horizon. Before it spreads, when a few rays appear to rise vertically from below upwards, that is the false dawn (subh kadhib). The false dawn is not valid for prayer nor for fasting; rather, only the true dawn is the actual dawn. This is what is meant by the clarification of the light.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 544