بسم الله الرحمن الرحيم
(Fatwa: 821/734=8/1438)
If one faces janabat(impurity due to discharge of semen) during a journey and no water is available and there is fear that the time of fardh salah will slip till one gets the water then one should do tayammum and offer salah.
(رد المحتار على الدر المختار 1/155 ط إحياء التراث) . بدائع الصنائع 1/46 ط المكتبة العلمية ، بيروت)
Note: It is mandatory to explain the reason of water being unavailable according to Fiqh. (D)
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband