The forgetting of the fish and Musa's retracing of his journey are pivotal to locating Al-Khidr, marking the designated meeting point.

According to Ibn Kathir, Musa was instructed by Allah to take a fish in a vessel, and its loss would signify reaching the servant of Allah (Al-Khidr) who possessed knowledge Musa sought (Source 1, 7). Upon reaching the "junction of the two seas" (مَجْمَعَ الْبَحْرَيْنِ - *majma' al-baḥrayn*), the fish came back to life and escaped into the sea, creating a tunnel-like path (سَرَبًا - *saraban*) (Source 1, 4). This event was the divinely appointed sign.

Musa's servant, Yusha' bin Nun, forgot to inform Musa about the fish's escape immediately, attributing his forgetfulness to Shaytan (Source 2, 3, 5). When Musa later felt fatigue and requested food, Yusha' recalled the incident, describing the fish's "strange way" (عَجَبًا - *'ajaban*) into the sea (Source 3, 5). Musa immediately recognized this as "what we have been seeking" (ذَلِكَ مَا كُنَّا نَبْغِ - *dhalika ma kunna nabghi*), confirming the sign (Source 5). Consequently, they "went back retracing their footsteps" (فَارْتَدَّا عَلَى ءَاثَارِهِمَا قَصَصًا - *fartadda 'ala atharihima qasasā*) to the rock where the fish had escaped, and there they found Al-Khidr (Source 2, 5). The forgetfulness, therefore, served as the mechanism to reveal the precise location for Musa's encounter with Al-Khidr.

📚 Sources: Classical tafsir (multi-source)