What is the reward for someone who suffers from flatulence and goes to the mosque just before the iqama? Does passing gas nullify ablution? And what should one do if gas is expelled during prayer?

If a worshipper's habit is to go early for prayer, and then he is delayed due to an excuse, it is hoped that he will be rewarded for going early, based on the hadith in Al-Bukhari: "If a servant falls ill or travels, Allah records for him the equivalent of what he used to do when he was healthy and resident." This applies to someone who was performing an act of obedience and was prevented from it, and whose intention was to continue it.

As for the invalidation of ablution during prayer, the original ruling is to exit the prayer and repeat the ablution. However, if the emission of gas is frequent and does not stop for a sufficient time to pray with a valid purification, then the worshipper is considered to have chronic flatulence ( سلس ريح ). In this case, he should perform ablution for each prayer after its time has begun, and then pray the obligatory and supererogatory prayers; what is emitted from him will not harm him.