What is required of a young man who covenanted with Allah in his prostration not to commit Zina, then broke his covenant, and is this covenant considered a solemn pledge?

One must fulfill a covenant made with Allah, especially if it involves abandoning the forbidden (haram). If one breaks such a covenant, they must repent, and if the covenant was verbally articulated, they must offer an expiation for breaking an oath (kaffarat yameen). This involves feeding ten poor people, clothing them, or emancipating a slave. If unable to do these, then one must fast for three days. However, if it was merely a fleeting thought (حديث نفس), then nothing is incumbent upon them. Adultery (zina) is one of the gravest sins, and its perpetrator must offer sincere repentance and desist from it. The door of repentance is open for anyone who sins and repents, even if the sin and repentance are repeated.