The meaning of the words: “Acts of worship are tawqeefi”
Question

What does “Acts of worship are tawqeefi” mean?.
Praise be to Allah.What is meant when the scholars say that acts of worship are tawqeefi or acts of worship are based on tawqeef is that it is not permissible to worship Allah, may He be exalted, through any act of worship unless this act of worship is proven in the shar‘i texts (Qur’aan and Sunnah) to be an act of worship that was prescribed by Allah. 
No act of worship can be sanctioned except on the basis of shar‘i evidence to that effect. 
Allah, may He be glorified and exalted, says (interpretation of the meaning):
“This day, I have perfected your religion for you, completed My Favour upon you, and have chosen for you Islam as your religion”
[al-Maa’idah 5:3].
Allah, may He be exalted, has completed the religion for us; whatever Allah has not prescribed is not part of Islam. 
It was narrated from Abu Dharr (may Allah be pleased with him) that the Messenger of Allah (blessings and peace of Allah be upon him) said: “There is nothing by means of which one may draw close to Paradise and move far away from Hell but it has been explained to you.
Narrated by at-Tabaraani in al-Kabeer (1647) and classed as saheeh by al-Albaani in as-Saheehah, 1803 
Whatever the Messenger (blessings and peace of Allah be upon him) did not explain to us is not part of the religion and is not something by means of which one may draw close to Paradise and move far away from Hell. 
Shaykh al-Islam Ibn Tamiyah (may Allah have mercy on him) said: 
“From examining the principles of sharee‘ah we know that the acts of worship that have been enjoined by Allah or that He loves cannot be proven except through sharee‘ah. With regard to customs, they are things that people are used to doing in their worldly affairs … The basic principle concerning them is that they are not forbidden; nothing of that is forbidden except what Allah and His Messenger have forbidden. That is because commands and prohibitions have to do with the religion of Allah, and acts of worship must necessarily be enjoined. If something is not proven to have been enjoined, then how can it be deemed an act of worship? With regard to customs, if there is no text to prove that a custom is forbidden, how can it be deemed to be forbidden?  
Hence Ahmad and other scholars of hadeeth used to say: The basic principle with regard to acts of worship is tawqeef; no acts of worship are prescribed except that which has been prescribed by Allah, may He be exalted; if we do not adhere to this principle, we will be included among those referred to in the verse (interpretation of the meaning): “Or have they partners with Allah (false gods), who have instituted for them a religion which Allah has not allowed?” [ash-Shoora 42:21]. The basic principle with regard to customs is that they are permissible and none of them are forbidden except that which Allah has forbidden. If we do not adhere to this principle, we will be included among those referred to in the verse (interpretation of the meaning): “Tell me, what provision Allah has sent down to you! And you have made of it lawful and unlawful” [Yoonus 10:59].  
Hence Allah condemned the mushrikeen who introduced things into religion that Allah had not permitted and they forbade things that He had not forbidden. End quote. 
Majmoo‘ al-Fataawa, 29/16-17 
Shaykh Saalih al-Fawzaan (may Allah preserve him) said: 
Acts of worship are tawqeefi (i.e., acts of worship are to be based solely on Qur’aan and saheeh Sunnah, with no room for personal opinion); it is not permissible to do any act of worship in terms of time or place or type of worship except on the basis of tawqeef and instructions from the Lawgiver. As for the one who introduces something that was not enjoined by the Lawgiver of acts of worship or in terms of their place, time or description, then it is an innovation (bid ah). End quote. 
Al-Muntaqa min Fataawa al-Fawzaan, 16/13 
And Allah, may He be exalted, knows best.