First recorded by The Moonglows (US#20/R&B #1 1955).
Other hit versions by The McGuire Sisters (US #1/UK #14 1955), The Forrester Sisters (C&W #8 1989).
https://youtu.be/lZv3atM2arc
From the wiki: “Among the most seminal R&B and doo-wop groups of all time, The Moonglows’ lineup featured some of the genre’s greatest pure singers. The group were originally called ‘The Crazy Sounds’, but were renamed by radio disc-jockey Alan Freed as ‘The Moonglows’. (The group also cut some recordings as The Moonlighters.) Their first major hit was the #1 R&B gem ‘Sincerely’, for Chess Records, penned by group member Harvey Fuqua and recorded by them in late 1954.