Source: Man and Universe –by: Ayatollah Murtaza Mutahhari - p. 71-72
Unity of His work means to recognize that the world with all its systems, ways, causes and effects is the work of Allah alone and has originated from His will. Nothing in the world is self-existing. Everything depends on Him. In the words of the Holy Qur'an, He is the sustainer of the whole world. The existing things are not independent with regard to their effect and causation. As a result, as Allah has no partner in his essence, similarly He has no partner in His work. Every agent and every cause owes its existence and effectiveness to Him and depends on Him. All power as well as ability to do things belong to Him alone.
Man is one of the existing things and hence a creation of Allah. Like other things he is effective as far as his own work is concerned and unlike of them, he is the master of his own destiny. But Allah has in no way delegated His powers to him. Thus he has not got complete discretion.
"By the power of Allah I stand and sit".
The belief that any being, whether man or a being other than man, has a complete discretion, amounts to believing that being to be a partner of Allah as far as independence of activity is concerned. As independence in activity amounts to independence in essence it is contrary to Allah's unity of essence, what to say of His unity of work.
"All praise is due to Allah who has taken no spouse nor a child. He has no partner in His sovereignty, nor has He a helper to help Him out of weakness. Therefore glorify Him a great deal." (Surah Bani Isra'il, 17:111)