Is it permissible to link the response to a supplication (du'a') with the presence of sincerity (ikhlas) in one's actions, such that the response serves as proof of sincerity and its absence as proof of a lack thereof?

It is permissible for a Muslim to supplicate with the words: "O Allah, if I am sincere in this work of mine, then make it successful for me, or complete it for me." This is based on the hadith of the three men upon whom the rock fell, and they supplicated to Allah by virtue of their righteous deeds, saying: "O Allah, if I did that seeking Your countenance, then relieve us from what we are in." However, the success or completion of the work does not necessitate the complete sincerity of the supplicant. He may be sincere and yet be denied an answer due to a punishment or another reason, or he may be insincere and his work is completed as a form of gradual enticement (istidraj) or due to the alignment of destiny.