It is narrated from Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) that he said: I feel something (madhi) coming down from me like khazirah (a thin flour-and-meat porridge). When any of you finds that, he should wash his private part and perform ablution as for prayer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1667