I say, with Allah's guidance: It is recommended to start it when it is bright, and brightness is when the dawn has fully illuminated, such that one can recite forty verses or more, then repeat the prayer if it turns out that the ablution was invalid. Brightness at Fajr is recommended in both travel and residence, in summer and winter, except on the day of Muzdalifah, where praying early is better. As narrated by Rafi' ibn Khadij, Abu Huraira, Bilal, Anas, Ibn Mas'ud, and others, may Allah be pleased with them, the Prophet (peace be upon him) said: "Perform Fajr when it is bright, for it is greater in reward," in Sahih Ibn Hibban 4: 357, Jami' al-Tirmidhi 1: 289, and he said: Hasan Sahih, Sunan al-Nasa'i 1: 478, Majma' al-Zawa'id 1: 315, al-Ahad wa al-Mathani 1: 178, al-Mu'jam al-Kabir 4: 289, Musannaf Ibn Abi Shaybah 1: 284, Sharh Ma'ani al-Athar 1: 178, among others. Refer to: al-Dirayah 1: 103-104. Ibrahim al-Nakha'i, may Allah have mercy on him, said: "The companions of the Messenger of Allah (peace be upon him) did not agree on anything as they agreed on brightness," in Musannaf Ibn Abi Shaybah 1: 284, al-Athar 1: 20, 50, Sharh Ma'ani al-Athar 1: 184. Al-Zayla'i said in Nasb al-Rayah 1: 239: Its chain is authentic. Imam al-Tahawi said in Sharh Ma'ani al-Athar 1: 184: "It is not correct for them to agree on something contrary to what the Messenger of Allah (peace be upon him) did." Refer to: Sharh al-Wiqayah p.137, Tabyin al-Haqa'iq 1: 82, and Allah knows best.