'Umar said: 'A good bid'ah is this.
'Umar said: 'A good bid'ah is this.'" 'Allamah Qastalani writes in Irshad as-Sari (Commentary of Sahih al-Bukhari): He called it bid'ah, because the Messenger of Allah had not prescribed it for them nor was it in the days of the (first caliph) Siddiq (R.A), nor was it for the early parts of night, nor to this (fixed) number." Imam as-Suyuti (in Tarikhu 'I-Khulafa) Muhammad ibn Sa'd (in at-Tabaqat) and others have clearly counted the Taraweeh among the things started by 'Umar.
'Umar started it in Ramadhan, 15 A.H., the second year of his caliphate. Q. 82: SPECIAL NAWAFIL FOR THE MONTH OF RAMADHAN: Do we Shi 'as have special prayers during the Holy month of Ramadhan other than the Wajib 5 prayers? A: According to our traditions, there are 1000 rak'ah prayers (i.e. 500 prayers of 2 rak'ah each) spread over the 30 nights of the holy month.
The details are as follows: From 1st to 20th night: 20 rak'ah every night (8 rak'ah after Maghrib and 12 after 'Isha) Total: 400 rak'ah. From 21st to the end of month: 30 rak'ah every night (8 after Maghrib and 22 after 'Isha) Total: 300 rak'ah. 19th, 21st and 23rd nights: 100 raka'ah each night Total: 300 raka'ah. Thus 400+300+300=1000 rak'ah. It is to be prayed NOT in Jama'at because Nafilah prayer is not with Jama'at.
This was the mistake made by 'Umar (in Taraweeh) that he made people pray nawafil with Jama'at. Q. 83: DECLARATION OF NEW MOON BY MUJTAHID'S ORDER: The first date of a month is not proved by the order of a Mujtahid and it is better to observe precaution." (Mas'alahNo. 1740). Please explain for me what does this Fatwa mean. A: It is a Mujtahid's prerogative to find out a rule of Shari'ah and make it known.
But when it comes to applying that rule in practical life, his words have the same value as those of any other learned and pious person. For example, Mujtahid has the authority to give fatwa that beer is haram and sherbat is halal but if there is a glass of coloured liquid and you do not know whether it is beer or sherbet, it is not a mujtahid's job to decide its reality. It is for the man who intends to use it to make sure that it is sherbet, not beer.
And even if a Mujtahid says so something about it, it has the same force as that of other knowledgeable persons. Coming to your question, the mujtahid has the authority to describe how the new moon should be proved.