It is narrated from Imam al-A'raj (may Allah have mercy on him) that he said: "I saw Sayyiduna Abu Hurayrah (may Allah be pleased with him) fall (so deeply) asleep while sitting that I could hear the sound of his snoring; then he would get up and pray, and he would not perform ablution (again)."
Takhrij and grading: The chain of this report (athar) is sahih (authentic). This narration indicates that sleep while sitting firmly does not break ablution.
Hadith Referenceالمطالب العالية بزوائد المسانيد الثمانية / كتاب الطهارة / 145
Hadith Gradingمحدثین:إسناده حسن
Hadith Takhrij«حکم الحدیث المطالب العالیہ محققاً تنسیق د. سعد بن ناصر بن عبد العزيز الشَّثري- مکتبہ شاملہ :إسناده حسن لغيره. » «انفرد به المصنف من هذا الطريق»