It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Companions said: "O Messenger of Allah! You become uncertain (in the prayer)." He (peace and blessings be upon him) said: "Why should I not become uncertain, when dirt is lodged between the nails and fingertips of one of you?"
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10363