This hadith has been narrated on the authority of Sulaiman Taimi with the same chain of transmitters and, at the end, it was said that the first Adhan was meant to awaken those who were in slumber amongst them and in order to make them turn who stand in (prayer) among them (towards food at the commencement of the fast). Jarir (one of the narrators) said that the Messenger ﷺ did not say like this but he said like it (true dawn) that the streaks of (true dawn ) are horizontal and not vertical.  

عَنْ سُلَيْمَانَ التَّيْمِيِّ بِهَذَا الإِسْنَادِ وَانْتَهَى حَدِيثُ الْمُعْتَمِرِ عِنْدَ قَوْلِهِ يُنَبِّهُ نَائِمَكُمْ وَيَرْجِعُ قَائِمَكُمْ وَقَالَ إِسْحَاقُ قَالَ جَرِيرٌ فِي حَدِيثِهِ وَلَيْسَ أَنْ يَقُولَ هَكَذَا وَلَكِنْ يَقُولُ هَكَذَا يَعْنِي الْفَجْرَ هُوَ الْمُعْتَرِضُ وَلَيْسَ بِالْمُسْتَطِيلِ  

[Machine] The Messenger of Allah ﷺ said, "Let no one prevent the call to prayer (Adhan) by Bilal, or he said, the announcement by Bilal, from his pre-dawn meal (Sahur), for indeed he gives the Adhan or he said, announces to notify the people who are standing or to wake up those who are sleeping." Then he said, "It is not appropriate for him to say like this, or to say like that, until he says like this."  

قَالَ رَسُولُ اللهِ ﷺ لَا يَمْنَعَنَّ أَحَدًا مِنْكُمْ أَذَانُ بِلَالٍ أَوْ قَالَ نِدَاءُ بِلَالٍ مِنْ سَحُورِهِ فَإِنَّهُ يُؤَذِّنُ أَوْ قَالَ يُنَادِي لِيَرْجِعَ قَائِمُكُمْ أَوْ لِيُنَبِّهَ نَائِمَكُمْ ثُمَّ قَالَ لَيْسَ أَنْ يَقُولَ هَكَذَا أَوْ قَوْلَ هَكَذَا حَتَّى يَقُولَ هَكَذَا