Musnad Ahmad 32:92

Ibn Abi ‘Adi narrated to us, from Humayd and Yazid. Humayd informed us—the meaning [is the same]—from Anas ibn Malik, who said: The call to prayer was made, so everyone whose house was near the mosque stood up, and those whose families were in distant houses remained. The Messenger of Allah (ﷺ) was brought a stone vessel, but it was too small for him to spread his hand in it. He said: "So he gathered his fingers." He said: "Then the rest of them performed ablution." Humayd said: Anas was asked, "How many were they?" He said: "Eighty or more."