If a person errs in an utterance, even if he mistakenly utters disbelief, he will not be held accountable. This man uttered a statement of disbelief since an individual’s saying to his Lord, “You are my slave and I am Your Lord” is undoubtedly disbelief. But because it resulted MISTAKENLY, out of extreme joy – he erred not knowing what to say – he was excused. So, if a person makes a mistake in an utterance, a statement of disbelief, he will not be held responsible for it.
*_[Sheikh Ibn Al-ʿUthaymīn, Sharḥ Riyāḍ Aṣ-Ṣāliḥīn 1/99]_*
Translation : Dārussalām