Britney Spears got some quality time in with her teenage sons ... and shared a rare photo of them all together as a memento.
Britney posted the pic Monday of her and her 2 boys -- 15-year-old Sean and 14-year-old Jayden -- standing in an open field together ... and she looks happy as can be.
She marveled at how big her kids are now and how time flies, saying ... "it's very hard for any mama especially a mama with boys seeing them grow up so fast !!!!"
Brit's also very proud of her sons, adding ... "I'm extremely lucky because my two babies are such gentleman and so kind that I must have done something right !!!!"
The singer says she hasn't posted any photos of Sean or Jayden in a while because -- well, they're teenagers -- but they let her post this one ... including an edited version with a more intense background.