Sayyiduna Wabisah bin Ma'bad (may Allah be pleased with him) said: I asked the Messenger of Allah (peace and blessings be upon him) about everything, until I even asked him about the dirt that collects under the nails, and he (peace and blessings be upon him) said: "Leave what makes you doubt for what does not make you doubt."
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18619