With this chain of transmission, from Al-Husayn ibn Sa'id, from Uthman ibn Isa, from Sama'ah. He said: I asked Abu Abdullah عليه السلام about a man who notices blood on his garment but forgets to wash it until he has already prayed. Imam عليه السلام said: "He must repeat his prayer so that he pays attention to such matters in his clothing, as a consequence for his forgetfulness." I said: "What about someone who was unaware of it - does he need to repeat the prayer after discovering it?" Imam عليه السلام said: "No, but he should start fresh (perform new prayers going forward)."