Sayyiduna Wabisah bin Ma'bad (may Allah be pleased with him) narrates: I asked the Messenger of Allah (peace and blessings be upon him) about everything, until I even asked him (peace and blessings be upon him) about the dirt that collects under the nails. He (peace and blessings be upon him) said: "Leave what makes you doubt for what does not make you doubt."
Al-Tabarani recorded it in al-Mu'jam al-Kabir; its chain contains a narrator, Talhah bin Zayd al-Raqqi, whose weakness is agreed upon.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1226