وَقَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ : " إِذَا نُودِيَ لِلصَّلَاةِ ، صَلَاةِ الصُّبْحِ ، وَأَحَدُكُمْ جُنُبٌ ، فَلَا يَصُمْ يَوْمَئِذٍ " .
And the Prophet Muhammad (peace and blessings be upon him) said: When the call to prayer for Fajr is given and any one of you is in a state of major ritual impurity (junub), then he should not fast that day.