let us see how good ibn umar was in regards to memory

we find in musnad ahmad

Urwa says that once we were standing while taking support on home of Ayesha; and she was cleaning teeth with maswaak; and I could hear voice of that; I asked ibn umar: did holy prophet asws do umra in month of rajab? He replied? Yes. I told this to Ayesha and she replied that may allah have mercy on ibn umar, whenever holy prophet do any umra, he was there with him; however, he forgot that holy prophet did not do any umra in month of rajab

[musnad ahmad, urdu, vol 11, page 339]

Sheikh hamza says isnaad sahih
[musnad ahmad with takhreej of sheikh ahmad shakir and sheikh hamza, vol 17, page 549]

Sheikh shoaib says isnaad sahih on condition of muslim
[musnad ahmad with takhreej of sheikh shoaib, vol 42, page 121, narration25238 ]

so, as per ayesha, he forgot that holy prophet had not done any umra in rajab where as he was there with him on all occasions