(a) Sayyidah Aishah (may Allah be pleased with her) narrates that the Prophet (peace and blessings be upon him) said: "When someone's ablution is broken during the prayer, he should place his hand on his nose and leave."
(b) Hajjaj bin Muhammad and others followed him in narrating it from Hisham as connected (mawsul), whereas al-Thawri, Shu'bah, Za'idah, Ibn al-Mubarak, Shu'ayb bin Ishaq and Abdah bin Sulayman narrated it from Hisham bin Urwah, from his father, from the Prophet (peace and blessings be upon him) as mursal. Abu Isa al-Tirmidhi (may Allah have mercy on him) said: This (mursal version) is more sound than the hadith of al-Fadl bin Musa.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3378