So this report is not contradictory to the first report, because His عليه السلام saying, “And forgetting the prostration in the first two and the last two (rak‘ahs) is the same,” He عليه السلام only intended by it the case of leaving both prostrations together. Do you not see that what the report contained was only the ruling of someone who left both prostrations together? For Imam عليه السلام said: “If he remembers it after the bowing, he must repeat the prayer.” So were it not that what is meant by remembering the prostration is both of them together, it would not have been obligatory to repeat prayer, as explained:
Commentary #19