Hadith: There are two 'Fajrs'. As for the 'Fajr' that resembles the tale of a wolf, it is not a sign that prayer has become due or food has become forbidden to consume. And as for the 'Fajr' that takes a rectangular shape in the horizon, it is a sign that prayer has become due and food forbidden to consume

عن جابر بن عبد الله رضي الله عنهما قال: قال رسول الله صلى الله عليه وسلم : الفجر فَجْرَانِ: فأما الفَجْر الذي يكون كَذَنَبِ السِّرْحَانِ فلا تَحِلُّ الصلاة فيه ولا يٌحْرِّم الطعام، وأما الذي يذهب مُسْتَطِيلًا فِي الأُفُق فإنه يُحِلُّ الصلاة، ويُحْرِّم الطعام.
[صحيح] - [رواه الحاكم]

Jābir ibn ‘Abdullāh (may Allah be pleased with him) reported that the Messenger of Allah (may Allah’s peace and blessings be upon him) said: "There are two 'Fajrs'. As for the 'Fajr' that resembles the tale of a wolf, it is not a sign that prayer has become due or food has become forbidden to consume. And as for the 'Fajr' that takes a rectangular shape in the horizon, it is a sign that prayer has become due and food forbidden to consume."
Sahih/Authentic. - [Al-Haakim]

Explanation

The Messenger (may Allah's peace and blessings be upon him) divided the 'Fajr' into two types: 1. The false 'Fajr': It goes up in the sky like a vertical light that resembles the tale of a wolf. The tale of a wolf moves upwards. Then, the false 'Fajr' disappears and the sky becomes dark again. So, this 'Fajr' does not make the Fajr prayer due or render food impermissible in case of a person intending to fast. This is because it is not the real 'Fajr' at which the Fajr prayer becomes due and a fasting person should abstain from food and drink. 2. The true 'Fajr': It is rectangular in shape as it stretches in the horizon from the north to the south. No darkness follows it. Rather, the sky gradually becomes brighter and brighter until light spreads all over the horizon. This is the 'Fajr' at which the prayer becomes due and food and drink become impermissible to a fasting person. The difference in time between the two 'Fajrs' varies, with the time becoming longer and shorter occasionally. And the false 'Fajr' does not appear sometimes. The essential distinction between the two 'Fajrs' lies in the following points: 1. The true 'Fajr' stretches from the north to the south, while the false one does the opposite. 2. The true 'Fajr' is not followed by darkness in the sky, unlike the false 'Fajr'. 3. The light of the true 'Fajr' is connected to the horizon, while that of the false one is disconnected. These three are physical and natural differences. In terms of the Shariah, the false 'Fajr' does not render prayer due or food and drink impermissible to a person intending to fast. The true 'Fajr', on the other hand, indicates that the time of the prayer has come and that it is now impermissible for a fasting Muslim to eat or drink.