Justin and Hailey Bieber Enlist Daniel Caesar as Wedding Singer

Probably didn't take 'em more than one phone call, but Justin and Hailey Bieber locked down their first choice for a wedding singer -- and Daniel Caesar will be doing the honors.

Sources with direct knowledge of this weekend's ceremony tell us ... the R&B star will be on the mic when Mr. and Mrs. Bieber celebrate their union in South Carolina with family and friends.

Justin's keeping it Canadian by enlisting Daniel, but we're told it's more than national pride -- Daniel is Hailey AND Justin's fave singer right now. Daniel's debut album, "Freudian," went gold and spawned a #1 hit on the R&B charts ... "Best Part."

It's a good bet he'll be singing that one when the nuptials go down on the Carolina coastline in Palmetto Bluff. As we first reported, the couple finally settled on Sept. 30 for the big ceremony they've been trying to pull off.

It's a formality -- but an important one for them -- since they officially tied the knot last September in an NYC courthouse.

There were rumors Justin's pal, Ed Sheeran, would be performing.

While it's true Ed sang at Scooter Braun's 2014 wedding ... we're told there will be no encore for Hailey and Justin's big day. Nothing scheduled, anyway.

Gotta say ... the Biebers have great musical taste!!!