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
باب إِذَا ذَكَرَ فِي الْمَسْجِدِ أَنَّهُ جُنُبٌ يَخْرُجُ كَمَا هُوَ وَلاَ يَتَيَمَّمُ
Sahih al-Bukhari, Book of Bathing (Ghusl)(view book)
Sahih al-Bukhari 275
Narrated Abu Huraira:
Once the call (Iqama) for the prayer was announced and the rows were straightened. Allah's Apostle came out;
and when he stood up at his Musalla, he remembered that he was Junub. Then he ordered us to stay at our places and went to take a bath and then returned with water dropping from his head. He said, "Allahu-Akbar", and we all offered the prayer with him.
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 ﷺ فَلَمَّا قَامَ فِي مُصَلاَّهُ ذَكَرَ أَنَّهُ جُنُبٌ فَقَالَ لَنَا : " مَكَانَكُمْ ". ثُمَّ رَجَعَ فَاغْتَسَلَ، ثُمَّ خَرَجَ إِلَيْنَا وَرَأْسُهُ يَقْطُرُ، فَكَبَّرَ فَصَلَّيْنَا مَعَهُ. تَابَعَهُ عَبْدُ الأَعْلَى عَنْ مَعْمَرٍ عَنِ الزُّهْرِيِّ. وَرَوَاهُ الأَوْزَاعِيُّ عَنِ الزُّهْرِيِّ.