Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates: We used to wipe it off with grass and wool; he meant semen.
Imam al-Tabarani recorded it in al-Mu'jam al-Kabir, and its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1534