Is it obligatory to say "Insha'Allah" when informing about a future action, and is one sinful for omitting it?

Allah Almighty commanded His servants to say "Insha'Allah" (if Allah wills) when informing of their intention to do something in the future, and He forbade His Prophet (peace be upon him) from omitting it, saying: (And never say of anything, "Indeed, I will do that tomorrow," Except [that you add], "If Allah wills.") [Al-Kahf: 23-24]. Many scholars have held that this command is for recommendation, while others believe it is obligatory. The benefit of saying "Insha'Allah" lies in seeking blessings, facilitating the matter, attaining barakah, the servant realizing his own helplessness, and seeking assistance from his Lord.