Musnad Abu Ya'la al-Mawsili 3:135

Ishaq ibn Isma'il al-Taliqani narrated to us, Sufyan ibn 'Uyaynah narrated to us, from 'Amr ibn Dinar, that al-Hasan ibn Muhammad informed me that 'Ubayd Allah, the scribe of 'Ali, informed him that he heard 'Ali saying: "The Messenger of Allah (ﷺ) sent me, al-Zubayr, and al-Miqdad—Sufyan said: 'These are the horsemen of the believers'—and he said: 'Go until you reach Rawdat Khakh.'" Then he mentioned the rest of it.