It is narrated from Abu Idris al-Khawlani (may Allah have mercy on him) that if a nosebleed occurs during the prayer, he should go back and continue on what he has already prayed. Atiyyah said: Ibn Umar and Abu Salamah bin Abd al-Rahman wrote to Umayyah bin Khalid bin Usayd to that effect, and he read their letter out to us.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3390