Makkaṭa Jātaka (No.173)
Once the Bodhisatta was a learned brahmin of Kāsi, and, when his wife died, he retired with his son to the Himavā, where they lived the ascetic life. One day during a heavy shower of rain, a monkey, wishing to gain admission to the ascetics’ hut, put on the bark dress of a dead ascetic and stood outside the door. The son wished to admit him, but the Bodhisatta recognised the monkey and drove him a way. The boy is identified with Rāhula (J.ii.68 f).
The circumstances in which the story was related are given in the Uddālaka Jātaka (q.v.)