Sayyiduna Abu Ayyub (may Allah be pleased with him) narrates: Once I went out with the Prophet (peace and blessings be upon him) when the sun had set, or was about to set, and I had a vessel of water with me. The Prophet (peace and blessings be upon him) went to relieve himself, and I kept waiting for him until he (peace and blessings be upon him) came back, and I helped him (peace and blessings be upon him) perform ablution. The narrator then mentioned the complete hadith.
Imam al-Tabarani recorded this report in al-Mu'jam al-Kabir; its chain contains a narrator, Abd al-Aziz bin Aban, on whose weakness the hadith scholars are agreed.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1148