Hadith from At-Tirmidhi:

Abu Hurairah (May Allah be pleased with him) reported: The Messenger of Allah (Peace be upon him) said, “The first of man’s deeds for which he will be called to account on the Day of Resurrection will be Salah. If it is found to be perfect, he will be safe and successful; but if it is incomplete, he will be unfortunate and a loser. If any shortcoming is found in the obligatory Salah, the Glorious and Exalted Allah will command to see whether His slave has offered any voluntary Salah so that the obligatory Salah may be made up by it. Then the rest of his actions will be treated in the same manner.”
[At-Tirmidhi].