I practice fasting every Thursday and Friday. Somebody told me that I should be fasting Monday and Thursday instead which is Masnoon and therefore more rewarding. Is it true and authentic?
Assalam alaiykum brother, yes the advice is based upon authentic sunnah.
The weekly fasts are on Mondays and Thursdays, on which days fasting is mustahabb.
It was narrated that ‘Aa’ishah said:
"The Messenger of Allaah (peace and blessings of Allaah be upon him) was keen to fast on Mondays and Thursdays.” (al-Nasaa’i, 2320; classed as saheeh by al-Albaani in Saheeh al-Jaami’ al-Sagheer, 4827).
The Prophet (peace and blessings of Allaah be upon him) was asked about fasting on Mondays and Thursdays, and he said:
"Those are two days on which people’s deeds are shown to the Lord of the Worlds, and I want my deeds to be shown to Him when I am fasting.” (al-Nasaa’i, 2358; Ibn Maajah, 1740; Ahmad, 8161; classed as saheeh by al-Albaani in Saheeh al-Jaami’, 1583).
He was asked about fasting on Mondays and he said, “On that day I was born and on that day revelation came to me.” (Muslim, 1162).
With regards to fasting on Fridays, it is forbidden by the Messenger (peace and blessings be upon him):
Volume 3, Book 31, Number 205:Narrated Muhammad bin 'Abbas:I asked Jabir "Did the Prophet forbid fasting on Fridays?" He replied, "Yes." (Other narrators added, "If he intends to fast only that day.")
Volume 3, Book 31, Number 206:Narrated Abu Huraira:I heard the Prophet saying, "None of you should fast on Friday unless he fasts a day before or after it.
Volume 3, Book 31, Number 207:Narrated Abu Aiyub from Juwairiya bint Al-Harith:The Prophet visited her (Juwairiya) on a Friday and she was fasting. He asked her, "Did you fast yesterday?" She said, "No." He said, "Do you intend to fast tomorrow?" She said, "No." He said, "Then break your fast." Through another series of narrators, Abu Aiyub is reported to have said, "He ordered her and she broke her fast."
'Amr al-'Ashari reported that he heard the Messenger of Allah say:"Verily, Friday is an 'id for you, so do not fast on it unless you fastthe day before or after it." This is related by al-Bazzar with a goodchain.
The day of Friday is a kind of weekly 'id for Muslims and, therefore, it isprohibited to fast on that day. Most scholars say that this prohibition is oneof dislike, and not one of complete forbiddance.
The above is correct