Is the prayer of one who led it valid, if in the second rakʿah of the Dawn prayer, he recited the first three verses of Surat Al-Fatihah silently and then completed it audibly, relying on the Maliki school of thought?

The words of the Maliki school indicate that whoever leaves off vocalizing more than two verses in a vocal prayer, it is sunnah for him to perform the prostration of forgetfulness (sujud al-sahw) before the salam. Ad-Dusuqi paused regarding leaving off vocalizing three verses due to the absence of a clear text, but the more cautious approach is to prostrate for leaving off vocalization in more than two verses. The prayer is valid whether one leaves off vocalizing two verses or more, because vocalizing the recitation is a single sunnah, and leaving off the prostration of forgetfulness does not invalidate the prayer unless it is due to leaving off three sunnahs.