Musnad Ahmad 1012:86

'Abd al-Wahhab ibn 'Ata narrated to us, [he said] al-Hasan ibn Dhakwan informed us, from 'Abd al-Wahid ibn Qays, from 'Ubadah ibn al-Samit, from the Prophet, peace and blessings of Allah be upon him, that he said: "The *Abdal* (substitutes) in this nation are thirty, like Ibrahim, the Friend of the Most Merciful. Whenever a man among them dies, Allah replaces his place with another man." [Note: Regarding this, the hadith of 'Abd al-Wahhab contains words other than this, and it is *munkar* (denounced), meaning the hadith of al-Hasan ibn Dhakwan.]