Abdullah bin Mubarak عبد الله بن المبارك · The Book of the Masjids

"We went out as a delegation to the Prophet (ﷺ); we gave him our oath of allegiance and prayed with him. We told him that in our land there was a church that belonged to us. We asked him to give us the leftovers of his purification (Wudu' water). So he called for water, performed Wudu' and rinsed out his mouth, then he poured it into a vessel and said to us: 'Leave, and when you return to your land,…

"Let him spit to his left or beneath his left foot."