SQL Server 2017

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create procedure foo @bar integer as
select 'A' baz;
EXECUTE sp_helptext 'dbo.foo';

Text

create procedure foo @bar integer as
select 'A' baz;

SELECT m.definition
FROM sys.sql_modules AS m
WHERE m.object_id = OBJECT_ID('dbo.foo');

definition

create procedure foo @bar integer as
select 'A' baz;

SELECT OBJECT_DEFINITION(OBJECT_ID('dbo.foo'));

(No column name)

create procedure foo @bar integer as
select 'A' baz;

PRINT OBJECT_DEFINITION(OBJECT_ID('dbo.foo'));
create procedure foo @bar integer as
select 'A' baz;