I have researched the matter and found only this question (which seemed relevant but does not answer the exact question we're having): Quickened spells and concentration
Witch Bolt (PHB, p. 289) is a 1st-level sorcerer spell with a casting time of 1 action that requires concentration. The spell also states:
On a hit, the target takes 1d12 lightning damage, and on each of your turns for the duration, you can use your action to deal 1d12 lightning damage to the target automatically.
The Quickened Spell metamagic option for Sorcerers states simply:
When you cast a spell that has a casting time of 1 action, you can spend 2 sorcery points to change the casting time to 1 bonus action for this casting.
One of my players argue that Quickening the spell would allow them to use a bonus action to instantly attack the target on each subsequent turn.
Is there any official ruling on this anywhere? Is the difference between casting a spell and using the spell on subsequent turns while concentrating clearly explained anywhere?
I have no doubt that "casting the spell" and "using it while concentrating on it" operate differently and it would have to be specified in the metamagic feature if it had an impact on the concentration aspect of the spell, but I'm looking for an official written rule.
Thanks in advance.