the Fatwa Department Research Committee - chaired by Sheikh `Abd al-Wahhâb al-Turayrî
Ibn al-Jawzî relates in Manâqib al-Imâm Ahmad (420) that al-Warkânî – supposedly his neighbor – said: “On the day he died, twenty thousand Jews, Christians, and Magians embraced Islam.” Ibn al-Jawzî then mentions that in the narration of al-Warkânî’s claim that comes to us by way of Ibn Nu`aym, the number given is 10,000. Al-Dhahabî, in Mizân al-I`tidal, denies the veracity of this claim. He writes:
This al-Warkânî, his identity is unknown. Moreover, no one else ever came with a corroborating claim. If such a thing had happened, it would have been a major event and a lot of people would have narrated it. As for (the possibility of that narrator being) Muhammad b. Ja`far al-Warkânî – the teacher of al-Baghawî – he was an honest narrator. However, he had died some time before Ahmad.
Therefore, what we have here is a far-fetched and unsubstantiated claim about a mass conversion that, if true, would have been of historic proportions. And Allah knows best.
Source: Islam Today