Worshipping Allah by prostrating only
Question

My colleague is a religious brother he prays obligatory and Sunnah prayers. I found him once prostrating to Allah a single prostrating which is not the prostrating of prayers, forgetting, reciting, or thanking. It is prostration he does at any time. In this sajdah he mentions Allah by the narrated adhkaar of the prophet Muhammad (PBUH). His evidence for this is the Hadeeth “Assist me in helping you with much prostration”. Is this prostration correct or is it an innovation?.
Praise be to Allah.The basic principle with regard to acts of worship is tawqeef (i.e., doing nothing except that which is prescribed in the Quran and Sunnah). So Allah should not be worshipped except in the ways that He has prescribed. There is nothing in sharee’ah to suggest that He may be worshipped by prostrating only, unless it is sajdat tilaawah (prostration when reading Quran) or sajdat shukr (prostration of gratitude). Nothing else is prescribed. The Prophet (peace and blessings of Allah be upon him) said: “Whoever introduces anything into this matter of ours that is not part of it will have it rejected.” Narrated by al-Bukhaari (2499) and Muslim (3242). According to a version narrated by Muslim: “Whoever does any action that is not part of this matter of ours will have it rejected.” 
Al-Nawawi (may Allah have mercy on him) said in al-Majmoo’ (3/565): If a person humbles himself before Allah and seeks to draw near to Him by doing a prostration for a reason that does not dictate doing the prostration of gratitude, there are two views that were narrated by Imam al-Haramayn and others. The first is that it is permissible, which is the view of the author of al-Taqreeb. But the more correct view is that it is not permissible. This was classed as the correct view by Imam al-Haramayn and others, and was regarded as definitively correct by Shaykh Abu Haamid. Imam al-Haramayn said: My Shaykh – i.e., Abu Muhammad – denounced this prostration sternly, and they quoted as evidence for that the analogy with bowing. If a person were to worship Allah by bowing only, that is haraam according to scholarly consensus, because it is bid’ah (an innovation), and every innovation is a going astray, except in cases where there is evidence to the contrary, regardless of whether it is done after prayer or in any other situation. End quote. 
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: Worse than this is what some people do, when they do a single prostration after saying the salaam. This is an innovation and it was not narrated from any of the Imams that this is mustahabb. 
Acts of worship are based on sharee’ah and following, not on whims and desires or innovations. Islam is based on two principles: that we do not worship anyone but Allah alone, and we worship Him in the way that He prescribed on the lips of His Messenger (peace and blessings of Allah be upon him), and we do not worship Him on the basis of whims and desires or innovations. End quote from al-Fataawa al-Kubra (2/242). 
Ibn al-Haaj said in al-Madkhil (4/266), denouncing the two prostrations done after al-raghaa’ib prayer: 
Prayer is only meant as an act of worship to draw closer to Allah, and worship can only be by following, not innovating or doing anything makrooh. … The scholars allowed single prostrations outside of prayer in only two cases, not three. The first is prostration of recitation (when reading Quran) and the second is the prostration of gratitude according to the view of those who accept it. These two prostrations are not included in that because there is no report of that from the earlier generations (may Allah be pleased with them). End quote. 
It should be noted that the shaytaan is keen to make people fall into bid’ah (innovation), for several reasons: 
So that he will not be rewarded for these actions 
And so as to distract him from acting on the Sunnah, with the implication that the Lawgiver omitted something, as if the innovator has been guided to something to which Muhammad (peace and blessings of Allah be upon him) and his companions were not guided.  
To sum up: This prostration is not prescribed. This is indicated by the following: 
1-It is bid’ah, and the Prophet (peace and blessings of Allah be upon him) said: “Every innovation is a going astray.”
2-By analogy with the fact that it is not permissible to worship Allah by bowing only.
And Allah knows best.