If someone while in the mosque remembers that he is Junub, he should leave (the mosque to take a bath) and should not perform Tayammum

Sahih al-Bukhari · Book 5 · Hadith 275

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مُحَمَّدٍ، قَالَ حَدَّثَنَا عُثْمَانُ بْنُ عُمَرَ، قَالَ أَخْبَرَنَا يُونُسُ، عَنِ الزُّهْرِيِّ، عَنْ أَبِي سَلَمَةَ، عَنْ أَبِي هُرَيْرَةَ، قَالَ أُقِيمَتِ الصَّلاَةُ، وَعُدِّلَتِ الصُّفُوفُ قِيَامًا، فَخَرَجَ إِلَيْنَا رَسُولُ اللَّهِ صلى الله عليه وسلم فَلَمَّا قَامَ فِي مُصَلاَّهُ ذَكَرَ أَنَّهُ جُنُبٌ فَقَالَ لَنَا ‏:‏ ‏"‏…

Narrated Abu Huraira: Once the call (Iqama) for the prayer was announced and the rows were straightened. Allah's Messenger (ﷺ) came out; and when he stood up at his Musalla, he remembered that he was Junub. Then he orde…