Ibrahim bin Abd al-Rahman bin Awf said: At Siffin, on the day Sayyiduna Ammar (may Allah be pleased with him) was martyred, I heard him say, calling out loudly: "I am about to present myself before the All-Compelling (al-Jabbar), and I will be married to the wide-eyed maidens (al-hur al-'in), and today I will meet my beloved ones, Sayyiduna Muhammad (peace and blessings be upon him) and his party. The Prophet (peace and blessings be upon him) took a covenant from me that the last thing you take from this world will be thin milk mixed with water."
Al-Tabarani recorded it in al-Mu'jam al-Awsat, and Ahmad recorded it in abridged form. The narrators of both are the narrators of the Sahih, and al-Bazzar recorded something similar with a da'if (weak) chain.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب المناقب / 15608