What is the ruling on one who doubted his ablution after the second unit of prayer while he was an imam, and did not verify his doubt until he had completed the prayer?

The majority of jurists hold that doubt regarding the occurrence of an impurity (hadath) after having certainty of ablution (wudu') does not affect the validity of the ablution. This is based on the hadith: "He should not leave (his prayer) until he hears a sound or perceives a smell." Therefore, the prayer of the Imam and those behind him is valid. However, if one doubts the occurrence of ablution after having certainty of impurity, the prayer is invalid. The general rule is that certainty is not removed by doubt.