✅Answer: Please pay attention to the following narration:

1 عن عِيصِ بْنِ الْقَاسِمِ قَالَ: سَأَلْتُ أَبَا عَبْداللَّه عَنْ رَجُلٍ خَرَجَ فِي شَهْرِ رَمَضَانَ و أَصْحَابُهُ يَتَسَحَّرُونَ فِي بَيْتٍ فَنَظَرَ إِلَى الْفَجْرِ فَنَادَاهُمْ أَنَّهُ قَدْ طَلَعَ الْفَجْرُ فَكَفَّ بَعْضٌ وَ ظَنَّ بَعْضٌ أَنَّهُ يَسْخَرُ فَأَكَلَ فَقَالَ يُتِمُّ وَ يَقْضِي

📚 Al-Faqih, Vol. 2, p. 131

Ais ibn Qasim said: I asked Imam Sadiq (prayers of Allah be upon him): A person goes outside during Ramadan while his companions are having their predawn meal in a room. He looks at the dawn and calls out to them that the dawn has broken. Some of them stop eating, but one of them thinks that this person is joking and continues to eat! The Imam said: He should complete his fast and also make up for it!

2 عَنِ الْحَلَبِيِّ قَالَ: سَأَلْتُ أَبَا عَبْداللَّه عَنِ الْخَيْطِ الْأَبْيَضِ مِنَ الْخَيْطِ الْأَسْوَدِ فَقَالَ بَيَاضُ النَّهَارِ مِنْ سَوَادِ اللَّيْلِ قَالَ وَ كَانَ بِلَالٌ يُؤَذِّنُ لِلنَّبِيِّ وَ ابْنُ أُمِّ مَكْتُومٍ وَ كَانَ أَعْمَى يُؤَذِّنُ بِلَيْلٍ وَ يُؤَذِّنُ بِلَالٌ حِينَ يَطْلُعُ الْفَجْرُ فَقَالَ النَّبِيُّ إِذَا سَمِعْتُمْ صَوْتَ بِلَالٍ فَدَعُوا الطَّعَامَ وَ الشَّرَابَ فَقَدْ أَصْبَحْتُمْ

📚 Al-Kafi, Vol. 4, p. 98

Halabi said: I asked Imam Sadiq (prayers of Allah be upon him) about 'the white thread from the black thread' (in Surah Al-Baqarah, verse 178); the Imam said: (It means the appearance of) the whiteness of the day from the blackness of the night!

And he said: Bilal used to call the adhan for the Prophet (peace be upon him and his family), and Ibn Umm Maktum, who was blind, would call the adhan at night, and Bilal would call the adhan at the break of dawn! And the Prophet (peace be upon him and his family) would say: When you hear the voice (of the adhan) of Bilal, stop eating and drinking, for you have entered the morning!

3️⃣ From Ahmad ibn Abdullah Al-Gharawi from his father, he said: I entered upon Al-Fadl ibn Al-Rabi' while he was sitting on a roof. So he said to me: Come close. So I came close until I was beside him. Then he said to me: Look down into a room in the house. So I looked down. He said: What do you see in the room? I said: A piece of cloth thrown on the floor! He said: Look carefully. So I considered and looked and I was certain and I said: A man prostrating! ...until he said: He said: This is Abul-Hasan Musa ibn Ja'far (prayers of Allah be upon him) and I look for him day and night and I do not find him at any time except in the state that I have told you!

He indeed prays the dawn prayer and then performs post-prayer supplications for an hour until the sun rises, and then he prostrates and remains prostrating until the sun declines (at noon) and he has appointed someone to watch for the decline for him, and I do not know when the boy tells him: The sun has declined, for he jumps up and begins the prayer without making a new ablution. And I realize that he did not fall asleep or doze off during his prostration.....!

📚 Uyun Akhbar al-Reza, Vol. 1, p. 106

Abdullah Al-Gharawi said: I entered upon Al-Fadl ibn Al-Rabi' while he was sitting on a roof. So he said to me: Come close! I came close until I was beside him. Then he told me to look from that height into a room in a house, and I looked. He said to me: What do you see in that room?! I said: A piece of cloth that has fallen on the ground! He said: Look carefully and I focused and looked and became certain and I said: A man is in prostration!... until he said: This is Abul-Hasan Musa ibn Ja'far (prayers of Allah be upon him) and I seek him day and night and I do not find him at any time except in the state that I have informed you of!

He indeed prays the morning prayer and for a while after the prayer, he performs supplications until the sun rises, and then he goes into prostration and he remains prostrating until the sun declines and it becomes noon, and he has appointed a person to watch for the decline for him, and I do not know when the boy tells him: The sun has declined, for he suddenly rises and begins the prayer without renewing (his ablution) and I realize that he did not fall asleep or doze off in his prostration.....!

✅ Based on the above and other narrations, the adhan and announcement of even one trustworthy person are sufficient for certainty regarding the entry of prayer time and the necessity of abstaining from eating and drinking for fasting. It is necessary for a person to stop eating and drinking upon the statement of even one trustworthy person, and if someone in Ramadan assumes that the person is joking and continues to eat, he is obligated to fast that day and complete it and also make up for it.

اللَّهُمَّ فَصَلِّ عَلَی مُحَمَّدٍ وَ آلِ مُحَمَّدٍ وَ عَجِّلْ لِوَلِیِّکَ الْفَرَجَ وَ الْعَافِیَةَ وَ النَّصْرَ