Sulayman bin Musa (may Allah have mercy on him) said: I heard Salim (may Allah have mercy on him) narrating from his father (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "None should touch the Qur'an except one in a state of ablution."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 410