What was the first a cappella song to top the Billboard Hot 100?

I've always found a cappella music completely mesmerizing. You're telling me those drum sounds are from a mouth? Throughout music history, most a cappella hits stayed firmly in their lane, content to be novelty acts or doo-wop throwbacks. But one track managed to break through and prove that you didn't need a single instrument to dominate the pop charts. What was the first a cappella song to reach number one on the Billboard Hot 100?