(a) It is narrated from Sayyiduna Abdullah bin Umar bin al-Khattab (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Prayer is not accepted without purification, and charity is not accepted from stolen wealth."
(b) These hadiths establish that once a ritual impurity (hadath) occurs, it is obligatory to break off the prayer, and also to perform ablution. Since the prayer begins with the takbir, he should start the prayer afresh by saying the takbir anew.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3380