It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "We wished that Musa (peace be upon him) had been patient with him (with Khidr, peace be upon him), so that he would have told us more of the story of the two of them."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12248