Muhammad bin Amr bin Ata says: I saw Sayyiduna al-Sa'ib bin Khabbab (may Allah be pleased with him) smelling his garment, so I asked: What is the reason for this? May Allah Almighty have mercy on you! He replied: I heard the Prophet (peace and blessings be upon him) say: "Ablution becomes necessary upon (perceiving) the smell (of passed wind, or) hearing (its sound)."
Al-Tabarani recorded it in al-Mu'jam al-Kabir; its chain contains a narrator, Abd al-Aziz bin Ubaydullah, who is da'if al-hadith (weak in hadith), and I have not seen anyone declare him trustworthy. And Allah knows best.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1250
Hadith Gradingمحدثین:إسنادہ ضعیف
Hadith Takhrij«أخرجه سنن ابن ماجه برقم: 516، وأخرجه مسند أحمد برقم: 15080، وأخرجه المعجم الكبير للطبراني برقم: 6622»