As per the research we have conducted, not only there is not such a saying in our textual sources (hadith books) but to the contrary, we came across narrations which expressly indicate that the Holy Prophet (pbuh) and the imams (a.s.) ate grapes even at night. Example: 1. One of those narrations is the following: It has been said that the Prophet (pbuh) observed fast for forty days at God's order. He was wakeful and engaged in prayers for forty nights. After completion of forty days Michael came down carrying a tray covered with a woolen cloth. He put the tray in front of the Prophet (pbuh), and the angel went close to the Prophet (pbuh) and said: “O' Muhammad, God commanded me to tell you to break your fast today with this food.”
There was a bunch of dates and a bunch of grapes on the tray. The Messenger of God, peace be upon him and his family, ate enough from them and he also drank water thereupon …
2. One of the bondwomen of Imam Sajjad (a.s.) took some grapes to the Imam during Iftar time. The Imam liked it. For two consecutive nights, the Imam gave his share to a poor man. On the third night, the bondwoman again took the Imam (a.s.) some grapes. On that night, no beggar came along. The Imam (a.s.) ate the grapes…"
As for drinking grape juice, if the juice of grapes ferments by itself, or when heated, it becomes haraam. However, if it boils so much that only 1/3 of it is left, there would be no problem in drinking it. Similarly, if grape juice turns into vinegar, there would be no problem in drinking it. It is immaterial whether one drinks it at night time or during the day.
As per the research we have conducted, not only there is not such a saying in our textual sources but to the contrary, we came across narrations which expressly indicate that the infallible imams (a.s.) and the Holy Prophet (pbuh) ate grapes even at night. For example:
1. It is said that one day the Messenger of God (pbuh) was sitting at a place and he was accompanied by Ammar bin Yasir, Munzir bin Zahzāh, Abu Bakr, Umar, Ali bin Abi Talib, Abbas bin Abdul Muttalib and Hamza bin Abdul Muttalib when all of a sudden Gabriel descended in its mammoth shape spreading its wings to the east and the west and addressing Prophet Muhammad (pbuh) as such, “O’ Muhammad, God, the Exalted greets you and orders you to stay away from Khadija for forty days.”
It was difficult for the Holy Prophet to stay away from Khadija because he loved her very much and was greatly attached to her. The Prophet (pbuh) got up; he observed fast for forty days and stayed awake for forty nights worshipping and glorifying God, the Exalted. During the last days of seclusion, the Prophet (pbuh) dispatched Ammar bin Yasir to convey his message to Khadija. In his message to her, the Prophet (pbuh) said: “O’ Khadija, do not think that the reason behind my staying away from you is my resentment against you. It is all because God, the Exalted, commanded me to stay away and I am abiding by His order. Do not think except in a good manner in this regard for God, the Exalted, is proud of you to the angels. At night time, lock the door from behind and rest comfortably. And know that during these days I am in Fatima bint Asad’s house (mother of Ali bin Abi Talib and wife of the Prophet’s uncle).”
Khadija was sad and thoughtful during these days. After completion of forty days of seclusion, Gabriel came down upon the Prophet (pbuh) and said:
‘O Muhammad, God greets you and commands you to benefit from His gift.”
The Prophet inquired, “What is that gift of the Lord of the worlds? And what is His glad-tiding?
“I don’t know,” said Gabriel.
At this time, Michael descended carrying a tray covered with a woolen cloth. He put the tray in front of the Prophet (pbuh), and the angel approached the Prophet (pbuh) and said: O' Muhammad, God commands you to break your fast today with this food.”
Ali bin Abi Talib says, “Whenever the Prophet (pbuh) wanted to break his fast he would order me to open the door so that whoever wished to eat the food could come in but on this night, the Prophet (pbuh) put me at the door saying, “This is a special food; it is prohibited for anyone other than me. He then removed the cloth from the tray. There were a bunch of dates and a bunch of grapes on it. The Holy Prophet had enough from them and he had also a full drink of water…”
2. Imam Ja’far Sadiq (a.s.) said, “Imam Sajjad liked grapes. They brought some grapes to Medina and one of the Imam’s bondwomen bought some. She took it to him during the time for breaking fast. He liked the grapes. Before stretching his hand to the grapes to take some, a beggar came along. He ordered that all the grapes be given to him.
“A small quantity would suffice him,” said the bondwoman to the Imam.
“No, by Allah, take all of it,” Imam Sajjad said.
She took all of the grapes to the needy man. The next day, she again bought some grapes and brought home. That day also, a beggar came along and the entire grapes were given to him. The next day, again she bought some grapes for the Imam. On the third night, no beggar came along and the Imam ate the grapes…
As for drinking grape juice, jurisprudents and religious authorities have discussed this issue in their Manual of Islamic Laws. They say: If the juice of grapes ferments irrespective of whether it is heated or it ferments by itself because of the heat of the sun, it is harām to drink it until 2/3 of it evaporates and 1/3 part of it is left or until it turns into vinegar.
Of course, grape juice or grape water available in Muslim countries is treated as halal because it is sold on Muslim markets. Thus, with the observance of the conditions mentioned by grand religious authorities, there is no objection in drinking grape juice at day or night time. Also, there is no problem in drinking the grape juice imported from non-Muslim countries, if we know that it has fermented to the extent that 2/3 of it has evaporated and 1/3 is left or when it has never been boiled.
Late Ayatollah Kheoi was asked about imported grape water and grape juice sold on the market with a label “Boiled in intense heat” on their containers. It was enquired whether it was permissible to drink it if one knows that it has been heated or when one entertains doubt about it. He answered: “It is permissible in the said case.”
 - Majlisi, Behar al-Anwar, 16, pg.79, al-Wafa Institute, Beirut, 1404 A.H.
 - Hurr al-Ameli, Wasail al-Shi’ah, vol.25, pg.148 – 149, chapter 82, Aalulbayt, Qom, 1409 A.H.
 - Imam Khomeini, Tahrirul Wasilah, vol.2, pg. 478, Darul Ilm Institute, Qom, 1st edition. For further information in this regard, refer to Tauzihul Masail of Maraje’, vol.1, pg.122-124.
 - Musavi Khoei, Sayed Abul Qasim, Serat al-Nejat (with connotations by Tabrizi), vol.2, pg.39, al-Muntakhab Publication Office, - qom, 1st edition, 1416 A.H.