That incident was not forgotten by Brahma (that Brahma's previous sons the four kumaras, refused to obey their father), and therefore the obedience of Manu Svayambhuva was very encouraging

Expressions researched:
"That incident was not forgotten by Brahma, and therefore the obedience of Manu Svayambhuva was very encouraging"

SB Canto 3

When the four previous sons of Brahmā, the sages Sanaka, Sanātana, Sanandana and Sanat-kumāra, refused to obey their father, Brahmā was mortified, and his anger was manifested in the shape of Rudra. That incident was not forgotten by Brahmā, and therefore the obedience of Manu Svāyambhuva was very encouraging.

SB 3.13.10, Translation and Purport:

O hero, your example is quite befitting a son in relationship with his father. This sort of adoration for the superior is required. One who is beyond the limit of envy and who is sane accepts the order of his father with great delight and executes it to his full capacity.

When the four previous sons of Brahmā, the sages Sanaka, Sanātana, Sanandana and Sanat-kumāra, refused to obey their father, Brahmā was mortified, and his anger was manifested in the shape of Rudra. That incident was not forgotten by Brahmā, and therefore the obedience of Manu Svāyambhuva was very encouraging. From the material point of view, the four sages' disobedience to the order of their father was certainly abominable, but because such disobedience was for a higher purpose, they were free from the reaction of disobedience. Those who disobey their fathers on material grounds, however, are surely subjected to disciplinary reaction for such disobedience. Manu's obedience to his father on material grounds was certainly free from envy, and in the material world it is imperative for ordinary men to follow the example of Manu.