It is narrated from Sayyidah Busrah bint Safwan (may Allah be pleased with her) that the Prophet (peace and blessings be upon him) said: "When anyone touches his private part, let him not pray until he performs ablution." The narrator said that Urwah objected to this, then asked Sayyidah Busrah (may Allah be pleased with her), and she confirmed it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 623