He should not cease (to pray) unless he hears a sound or perceives a smell (of passing wind).  

شُكِيَ إِلَى النَّبِيِّ ﷺ الرَّجُلُ يَجِدُ الشَّىْءَ فِي الصَّلاَةِ حَتَّى يُخَيَّلَ إِلَيْهِ فَقَالَ لاَ يَنْفَتِلُ حَتَّى يَسْمَعَ صَوْتًا أَوْ يَجِدَ رِيحًا  

The Messenger of Allah ﷺ said: If any one of you offers prayer and feels a movement between his paddocks, but is doubtful whether or not his ablution broke, he should not cease praying unless he hears a sound or perceives a smell.  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ إِذَا كَانَ أَحَدُكُمْ فِي الصَّلاَةِ فَوَجَدَ حَرَكَةً فِي دُبُرِهِ أَحْدَثَ أَوْ لَمْ يُحْدِثْ فَأَشْكَلَ عَلَيْهِ فَلاَ يَنْصَرِفْ حَتَّى يَسْمَعَ صَوْتًا أَوْ يَجِدَ رِيحًا