التفسير التبليغي
(قَالُوا تَاللَّهِ إِنَّكَ لَفِي ضَلَالِكَ الْقَدِيمِ)
Yusuf's brothers were still on their way... and the brothers, as we know, their lives had changed, they had repented, returned [to Allah], regretted, and their character had improved.
(قَالُوا) meaning: his grandchildren.
Those who (قَالُوا تَاللَّهِ إِنَّكَ لَفِي ضَلَالِكَ الْقَدِيمِ) were the present members of his household.
(قَالُوا) meaning: his grandchildren and those with him, (تَاللَّهِ إِنَّكَ لَفِي ضَلَالِكَ الْقَدِيمِ) meaning: you are certainly astray from the right path in your excessive love for Yusuf, your constant mention of him, and your hope of meeting him, as they believed he had died.
Those who said this were not his sons, because his sons were traveling on their way to him.
They saw the old man inhaling Yusuf's fragrance, and this angered them. (قَالُوا تَاللَّهِ إِنَّكَ لَفِي ضَلَالِكَ الْقَدِيمِ).
His household said (تَاللَّهِ إِنَّكَ لَفِي ضَلَالِكَ) meaning: your old delusion, for imagining the return of what is lost.
(الْقَدِيمِ) meaning: your old error in thinking Yusuf is alive.
They do not find what Ya'qub finds, nor do they know what he knows, nor do they smell what he smells...!!
According to Qatadah: (قَالُوا تَاللَّهِ إِنَّكَ لَفِي ضَلَالِكَ الْقَدِيمِ) meaning: from your love for Yusuf, you neither forget him nor find solace from him.
They said a harsh word to their father, which they should not have said to Allah's Prophet (peace be upon him).
Al-Hasan said: And this is an act of disobedience.
*Ad-Dalal* (ضلال): being far from the path that leads [to the destination].
Source: التفسير التبليغي