Abd al-Karim Abu Umayyah says: Sayyiduna Ali (may Allah be pleased with him), Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) and al-Sha'bi said about a man who falls asleep while sitting: ablution is not obligatory upon him.
Al-Tabarani recorded it in al-Mu'jam al-Kabir. The narrator Abd al-Karim is da'if (weak), and he did not live in the time of Sayyiduna Ali (may Allah be pleased with him) or Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1291